btrfs: return ENOMEM if path allocation fails in btrfs_cross_ref_exist

The error code does not match the reason of failure and may confuse the
callers.

Signed-off-by: Su Yue <suy.fnst@cn.fujitsu.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>