nfsd: pass proper net to nfsd_destroy() from NFSd kthreads

Since NFSd service is per-net now, we have to pass proper network
context in nfsd_shutdown() from NFSd kthreads.

The simplest way I found is to get proper net from one of transports
with permanent sockets.

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