nfsd: simplify service shutdown

Function nfsd_shutdown is called from two places: nfsd_last_thread (when last
kernel thread is exiting) and nfsd_svc (in case of kthreads starting error).
When calling from nfsd_svc(), we can be sure that per-net resources are
allocated, so we don't need to check per-net nfsd_net_up boolean flag.
This allows us to remove nfsd_shutdown function at all and move check for
per-net nfsd_net_up boolean flag to nfsd_last_thread.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>