ceph: normalize 'delta' parameter usage in check_quota_exceeded

Function check_quota_exceeded() uses delta parameter only for the
QUOTA_CHECK_MAX_BYTES_OP operation.  Using this parameter also for
MAX_FILES will makes the code cleaner and will be required to support
cross-quota-tree renames.

Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>