btrfs: remove useless dio wait call when doing fallocate zero range

When starting a fallocate zero range operation, before getting the first
extent map for the range, we make a call to inode_dio_wait().

This logic was needed in the past because direct IO writes within the
i_size boundary did not take the inode's VFS lock. This was because that
lock used to be a mutex, then some years ago it was switched to a rw
semaphore (by commit 9902af79c0 ("parallel lookups: actual switch to
rwsem")), and then btrfs was changed to take the VFS inode's lock in
shared mode for writes that don't cross the i_size boundary (done in
commit e9adabb971 ("btrfs: use shared lock for direct writes within
EOF")). The lockless direct IO writes could result in a race with the
zero range operation, resulting in the later getting a stale extent
map for the range.

So remove this no longer needed call to inode_dio_wait(), as fallocate
takes the inode's VFS lock in exclusive mode and direct IO writes within
i_size take that same lock in shared mode.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/file.c

@@ -3237,8 +3237,6 @@ static int btrfs_zero_range(struct inode *inode,
 	u64 bytes_to_reserve = 0;
 	bool space_reserved = false;
 
-	inode_dio_wait(inode);
-
 	em = btrfs_get_extent(BTRFS_I(inode), NULL, 0, alloc_start,
 			      alloc_end - alloc_start);
 	if (IS_ERR(em)) {