btrfs: don't BUG_ON allocation errors in btrfs_drop_snapshot

In addition to properly handling allocation failure from btrfs_alloc_path, I
also fixed up the kzalloc error handling code immediately below it.
Signed-off-by: Mark Fasheh <mfasheh@suse.com>

fs/btrfs/extent-tree.c

@@ -6272,10 +6272,14 @@ int btrfs_drop_snapshot(struct btrfs_root *root,
 	int level;
 
 	path = btrfs_alloc_path();
-	BUG_ON(!path);
+	if (!path)
+		return -ENOMEM;
 
 	wc = kzalloc(sizeof(*wc), GFP_NOFS);
-	BUG_ON(!wc);
+	if (!wc) {
+		btrfs_free_path(path);
+		return -ENOMEM;
+	}
 
 	trans = btrfs_start_transaction(tree_root, 0);
 	BUG_ON(IS_ERR(trans));