Allow loading of dtraceall without nfscl if what you really wnat is nfsclient

Obtained from:	Panzura tree
MFC after:	1 week
Change loginclass mutex to an rwlock.

While here reduce nesting in loginclass_free.

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