nfsd: simplify NFSv4 state init and shutdown

This patch moves nfsd_startup_generic() and nfsd_shutdown_generic()
calls to nfsd_startup_net() and nfsd_shutdown_net() respectively, which
allows us to call nfsd_startup_net() instead of nfsd_startup() and makes
the code look clearer.  It also modifies nfsd_svc() and nfsd_shutdown()
to check nn->nfsd_net_up instead of global nfsd_up.  The latter is now
used only for generic resources shutdown and is currently useless.  It
will replaced by NFSd users counter later in this series.

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