fix mountd.c so that it does not allocate 4K for credential groups unless a large groups list is needed

[rmacklem]

Summary

r362163 upgraded mountd so that it could handle MAX_NGROUPS
groups for the anonymous user credentials (the ones provided by
-maproot and -mapall exports options).
The problem is that this resulted in every export structure growing by
about 4Kbytes, because the cr_groups field went from 16->MAX_NGROUPS.

This patch fixes this by only including a small 32 element cr_groups in the
structure and then malloc()'ng cr_groups when a larger one is needed.
The value of SMALLNGROUPS is arbitrarily set to 32, assuming most users
used by -maproot or -mapall will be in <= 32 groups.

Test Plan

Tested by doing an export entry with -maproot=ricktst,
when ricktst was in > SMALLNGROUPS groups.
For the test, syslog() calls were added to ensure the
malloc()s and free()s of the large cr_groups list
happened at the correct times.

[kib]

usr.sbin/mountd/mountd.c

1831 ↗

(On Diff #77339)

I find a test like anon.cr_groups != anon.cr_smallgrps more clean, instead of relying on the size comparision.

[freqlabs]

usr.sbin/mountd/mountd.c

3483 ↗

(On Diff #77339)

I take it this will not be in the committed version?

3528 ↗

(On Diff #77339)

How about using memcpy here, too?

[rmacklem]

Updated with changes as suggested by kib@ and freqlabs@.