cred: proc_set_cred(): Fix debug build

On stable/13, the 'cr_ref' field of 'struct ucred' is of type 'u_int',
contrary to stable/14 and main where it is of type 'long'.

This is a direct commit to stable/13.

Reported by: Jenkins
Fixes: b3d437832ffa ("cred: proc_set_cred(), proc_unset_cred(): Update user's process count")