ext4: fix buffer_head refcnt leak when ext4_iget() fails

ext4_orphan_get() invokes ext4_read_inode_bitmap(), which returns a
reference of the specified buffer_head object to "bitmap_bh" with
increased refcnt.

When ext4_orphan_get() returns, local variable "bitmap_bh" becomes
invalid, so the refcount should be decreased to keep refcount balanced.

The reference counting issue happens in one exception handling path of
ext4_orphan_get(). When ext4_iget() fails, the function forgets to
decrease the refcnt increased by ext4_read_inode_bitmap(), causing a
refcnt leak.

Fix this issue by calling brelse() when ext4_iget() fails.

Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Cc: stable@kernel.org
Link: https://lore.kernel.org/r/1587618568-13418-1-git-send-email-xiyuyang19@fudan.edu.cn
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ialloc.c

@@ -1246,6 +1246,7 @@ struct inode *ext4_orphan_get(struct super_block *sb, unsigned long ino)
 		ext4_error_err(sb, -err,
 			       "couldn't read orphan inode %lu (err %d)",
 			       ino, err);
+		brelse(bitmap_bh);
 		return inode;
 	}