nfsd: nfsd_destroy_serv() must call svc_destroy() even if nfsd_startup_net() failed

If nfsd_startup_net() fails and so ->nfsd_net_up is false,
nfsd_destroy_serv() doesn't currently call svc_destroy().  It should.

Fixes: 1e3577a4 ("SUNRPC: discard sv_refcnt, and svc_get/svc_put")
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>