nilfs2: fix incomplete buffer cleanup in nilfs_btnode_abort_change_key()

A syzbot fault injection test reported that nilfs_btnode_create_block, a
helper function that allocates a new node block for b-trees, causes a
kernel BUG for disk images where the file system block size is smaller
than the page size.

This was due to unexpected flags on the newly allocated buffer head, and
it turned out to be because the buffer flags were not cleared by
nilfs_btnode_abort_change_key() after an error occurred during a b-tree
update operation and the buffer was later reused in that state.

Fix this issue by using nilfs_btnode_delete() to abandon the unused
preallocated buffer in nilfs_btnode_abort_change_key().

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ryusuke Konishi <[e-mail]>
Reported-by: [e-mail]
Closes: https://lkml.kernel.org/r/[e-mail]
Tested-by: Ryusuke Konishi <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

fs/nilfs2/btnode.c

···
 	if (nbh == NULL) {	/* blocksize == pagesize */
 		xa_erase_irq(&btnc->i_pages, newkey);
 		unlock_page(ctxt->bh->b_page);
-	} else
-		brelse(nbh);
+	} else {
+		/*
+		 * When canceling a buffer that a prepare operation has
+		 * allocated to copy a node block to another location, use
+		 * nilfs_btnode_delete() to initialize and release the buffer
+		 * so that the buffer flags will not be in an inconsistent
+		 * state when it is reallocated.
+		 */
+		nilfs_btnode_delete(nbh);
+	}
 }