btrfs: consolidate the error handling in run_delalloc_nocow

Share the calls to extent_clear_unlock_delalloc for btrfs_path allocation
failure handling and the normal exit path.

This relies on btrfs_free_path ignoring a NULL pointer, and the
initialization of cur_offset to start at the beginning of the function.
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -1978,13 +1978,8 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,
 
 	path = btrfs_alloc_path();
 	if (!path) {
-		extent_clear_unlock_delalloc(inode, start, end, locked_page,
-					     EXTENT_LOCKED | EXTENT_DELALLOC |
-					     EXTENT_DO_ACCOUNTING |
-					     EXTENT_DEFRAG, PAGE_UNLOCK |
-					     PAGE_START_WRITEBACK |
-					     PAGE_END_WRITEBACK);
-		return -ENOMEM;
+		ret = -ENOMEM;
+		goto error;
 	}
 
 	nocow_args.end = end;