SUNRPC: Refactor svc_set_num_threads()

Refactor to separate out the functions of starting and stopping threads
so that they can be used in other helpers.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Tested-and-reviewed-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>