rpc_generic.c: Add CURVNET_SET/CURVNET_RESTORE around clnt_call_mbuf

[rmacklem]

Summary

During recent testing at a NFSv4 Bakeathon, I had a panic
that a vnet was not set in clnt_nl_call(). The call was done
from clnt_call_private() in rpc_generic.c.

Adding CURVNET_SET(TD_TO_VNET())/CURVNET_RESTORE
avoided further panics.

Test Plan

Tested while using NFSv4 mounts over TLS during a
recent NFSv4 Bakeathon testing event.

[rmacklem]

I realized that this is a NFS client side call,
so it needs to set vnet0.
I'll do that in the specific RPC-over-TLS code.