Btrfs: cleanup some BUG_ON()

This patch changes some BUG_ON() to the error return.
(but, most callers still use BUG_ON())

Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/inode-map.c

@@ -30,7 +30,8 @@ int btrfs_find_highest_inode(struct btrfs_root *root, u64 *objectid)
 	int slot;
 
 	path = btrfs_alloc_path();
-	BUG_ON(!path);
+	if (!path)
+		return -ENOMEM;
 
 	search_key.objectid = BTRFS_LAST_FREE_OBJECTID;
 	search_key.type = -1;