Use a temporary buffer in sys_setgroups for requests with <= XU_NGROUPS groups.

Submitted by:	Tiwei Bie <btw mail.ustc.edu.cn>
X-Additional: JuniorJobs project
MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273684 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f