cred: Separate constant for the number of inlined groups

CRED_SMALLGROUPS_NB now holds the number of inlined groups in field
'cr_smallgroups'. XU_NGROUPS stays the number of groups allowed in
'struct xucred'. The first is an implementation detail, while the
second is part of a public interface. All mentions of XU_NGROUPS in the
tree have been reviewed and only those concerning the implementation
detail have been changed to use CRED_SMALLGROUPS_NB (they were all in
'kern_prot.c').

No functional change, as CRED_SMALLGROUPS_NB is set to 16, the same
value as XU_NGROUPS.

Reviewed by: mhorne (slightly different version)
Approved by: markj (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D46911