Fix the server side krpc so that the kernel nfsd threads terminate.

Fix the server side krpc so that the kernel nfsd threads terminate.

Occationally the kernel nfsd threads would not terminate when a SIGKILL
was posted for the kernel process (called nfsd (slave)). When this occurred,
the thread associated with the process (called "ismaster") had returned from
svc_run_internal() and was sleeping waiting for the other threads to terminate.
The other threads (created by kthread_start()) were still in svc_run_internal()
handling NFS RPCs.
The only way this could occur is for the "ismaster" thread to return from
svc_run_internal() without having called svc_exit().
There was only one place in the code where this could happen and this patch
stops that from happening.
Since the problem is intermittent, I cannot be sure if this has fixed the
problem, but I have not seen an occurrence of the problem with this patch
applied.

Reviewed by:	kib
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D16087