Modify mountd.c so that it does not always malloc 4K for the map credentials.

Modify mountd.c so that it does not always malloc 4K for the map credentials.

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.

Reviewed by:	kib, freqlabs
Differential Revision:	https://reviews.freebsd.org/D26521