sunrpc: don't take ->sv_lock when updating ->sv_nrthreads.

As documented in svc_xprt.c, sv_nrthreads is protected by the service
mutex, and it does not need ->sv_lock.
(->sv_lock is needed only for sv_permsocks, sv_tempsocks, and
sv_tmpcnt).

So remove the unnecessary locking.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>