NFSD: move nfsd_mutex handling into nfsd_svc callers

Currently nfsd_svc holds the nfsd_mutex over the whole function. For
some of the later netlink patches though, we want to do some other
things to the server before starting it. Move the mutex handling into
the callers.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>