cred: fix struct credbatch to use long for refcount

This structure collects count from multiple cred structures. Of course it
can't use a smaller type.

PR: 283747
Reviewed by: olce, mjg, markj
Differential Revision: https://reviews.freebsd.org/D49562
Fixes: 37337709d3334f32650ba3a7c529fa013ed5e1f2