f2fs: fix to initialize variable to avoid UBSAN/smatch warning

As Dan Carpenter as below:

The patch df634f444ee9: "f2fs: use rb_*_cached friends" from Oct 4,
2018, leads to the following static checker warning:

	fs/f2fs/extent_cache.c:606 f2fs_update_extent_tree_range()
	error: uninitialized symbol 'leftmost'.

And also Eric Biggers, and Kyungtae Kim reported, there is an UBSAN
warning described as below:

We report a bug in linux-4.20.2: "UBSAN: Undefined behaviour in
fs/f2fs/extent_cache.c"

kernel config: https://kt0755.github.io/etc/config_v4.20_stable
repro: https://kt0755.github.io/etc/repro.4a3e7.c (f2fs is mounted on
/mnt/f2fs/)

This arose in f2fs_update_extent_tree_range (fs/f2fs/extent_cache.c:605).
It seems that, for some reason, its last argument became "24"
although that was supposed to be bool type.

=========================================
UBSAN: Undefined behaviour in fs/f2fs/extent_cache.c:605:4
load of value 24 is not a valid value for type '_Bool'
CPU: 0 PID: 6774 Comm: syz-executor5 Not tainted 4.20.2 #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0xb1/0x118 lib/dump_stack.c:113
 ubsan_epilogue+0x12/0x94 lib/ubsan.c:159
 __ubsan_handle_load_invalid_value+0x17a/0x1be lib/ubsan.c:457
 f2fs_update_extent_tree_range+0x1d4a/0x1d50 fs/f2fs/extent_cache.c:605
 f2fs_update_extent_cache+0x2b6/0x350 fs/f2fs/extent_cache.c:804
 f2fs_update_data_blkaddr+0x61/0x70 fs/f2fs/data.c:656
 f2fs_outplace_write_data+0x1d6/0x4b0 fs/f2fs/segment.c:3140
 f2fs_convert_inline_page+0x86d/0x2060 fs/f2fs/inline.c:163
 f2fs_convert_inline_inode+0x6b5/0xad0 fs/f2fs/inline.c:208
 f2fs_preallocate_blocks+0x78b/0xb00 fs/f2fs/data.c:982
 f2fs_file_write_iter+0x31b/0xf40 fs/f2fs/file.c:3062
 call_write_iter include/linux/fs.h:1857 [inline]
 new_sync_write fs/read_write.c:474 [inline]
 __vfs_write+0x538/0x6e0 fs/read_write.c:487
 vfs_write+0x1b3/0x520 fs/read_write.c:549
 ksys_write+0xde/0x1c0 fs/read_write.c:598
 __do_sys_write fs/read_write.c:610 [inline]
 __se_sys_write fs/read_write.c:607 [inline]
 __x64_sys_write+0x7e/0xc0 fs/read_write.c:607
 do_syscall_64+0xbe/0x4f0 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x4497b9
Code: e8 8c 9f 02 00 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48
89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d
01 f0 ff ff 0f 83 9b 6b fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f1ea15edc68 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f1ea15ee6cc RCX: 00000000004497b9
RDX: 0000000000001000 RSI: 0000000020000140 RDI: 0000000000000013
RBP: 000000000071bea0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 000000000000bb50 R14: 00000000006f4bf0 R15: 00007f1ea15ee700
=========================================

As I checked, this uninitialized variable won't cause extent cache
corruption, but in order to avoid such kind of warning of both UBSAN
and smatch, fix to initialize related variable.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reported-by: Eric Biggers <ebiggers@google.com>
Reported-by: Kyungtae Kim <kt0755@gmail.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/extent_cache.c

@@ -506,7 +506,7 @@ static void f2fs_update_extent_tree_range(struct inode *inode,
 	unsigned int end = fofs + len;
 	unsigned int pos = (unsigned int)fofs;
 	bool updated = false;
-	bool leftmost;
+	bool leftmost = false;
 
 	if (!et)
 		return;