Btrfs: cleanup BUG_ON in merge_bio

One can use btrfs-corrupt-block to hit BUG_ON() in merge_bio(),
thus this aims to stop anyone to panic the whole system by using
 their btrfs.

Since the error in merge_bio can only come from __btrfs_map_block()
when chunk tree mapping has something insane and __btrfs_map_block()
has already had printed the reason, we can just return errors in
merge_bio.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -2756,7 +2756,6 @@ static int merge_bio(int rw, struct extent_io_tree *tree, struct page *page,
 	if (tree->ops && tree->ops->merge_bio_hook)
 		ret = tree->ops->merge_bio_hook(rw, page, offset, size, bio,
 						bio_flags);
-	BUG_ON(ret < 0);
 	return ret;
 
 }

fs/btrfs/inode.c

@@ -1822,6 +1822,10 @@ static void btrfs_clear_bit_hook(struct inode *inode,
 /*
  * extent_io.c merge_bio_hook, this must check the chunk tree to make sure
  * we don't create bios that span stripes or chunks
+ *
+ * return 1 if page cannot be merged to bio
+ * return 0 if page can be merged to bio
+ * return error otherwise
  */
 int btrfs_merge_bio_hook(int rw, struct page *page, unsigned long offset,
 			 size_t size, struct bio *bio,
@@ -1840,8 +1844,8 @@ int btrfs_merge_bio_hook(int rw, struct page *page, unsigned long offset,
 	map_length = length;
 	ret = btrfs_map_block(root->fs_info, rw, logical,
 			      &map_length, NULL, 0);
-	/* Will always return 0 with map_multi == NULL */
-	BUG_ON(ret < 0);
+	if (ret < 0)
+		return ret;
 	if (map_length < length + size)
 		return 1;
 	return 0;