ext4: remove ext4_mb_return_to_preallocation()

This function was never implemented, except for a BUG_ON which was
tripping when ext4 is run without a journal.  The problem is that
although the comment asserts that "truncate (which is the only way to
free block) discards all preallocations", ext4_free_blocks() is also
called in various error recovery paths when blocks have been
allocated, but for various reasons, we were not able to use those data
blocks (for example, because we ran out of memory while trying to
manipulate the extent tree, or some other similar situation).

In addition to the fact that this function isn't implemented except
for the incorrect BUG_ON, the single caller of this function,
ext4_free_blocks(), doesn't use it all if the journal is enabled.

So remove the (stub) function entirely for now.  If we decide it's
better to add it back, it's only going to be useful with a relatively
large number of code changes anyway.

Google-Bug-Id: 3236408

Cc: Jiaying Zhang <jiayingz@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -3881,19 +3881,6 @@ repeat:
 	}
 }
 
-/*
- * finds all preallocated spaces and return blocks being freed to them
- * if preallocated space becomes full (no block is used from the space)
- * then the function frees space in buddy
- * XXX: at the moment, truncate (which is the only way to free blocks)
- * discards all preallocations
- */
-static void ext4_mb_return_to_preallocation(struct inode *inode,
-					struct ext4_buddy *e4b,
-					sector_t block, int count)
-{
-	BUG_ON(!list_empty(&EXT4_I(inode)->i_prealloc_list));
-}
 #ifdef CONFIG_EXT4_DEBUG
 static void ext4_mb_show_ac(struct ext4_allocation_context *ac)
 {
@@ -4648,7 +4635,6 @@ do_more:
 		ext4_lock_group(sb, block_group);
 		mb_clear_bits(bitmap_bh->b_data, bit, count);
 		mb_free_blocks(inode, &e4b, bit, count);
-		ext4_mb_return_to_preallocation(inode, &e4b, block, count);
 	}
 
 	ret = ext4_free_blks_count(sb, gdp) + count;