nlm: set vnet(9) context in the NLM syscall

With the kernel RPC binding moving to Netlink transport all clients need
to have proper vnet(9) context set. This change will unlikely make NLM
properly virtualized, but at least it will not panic on the default VNET
when kernel is compiled with VIMAGE.

Reviewed by: rmacklem
Differential Revision: https://reviews.freebsd.org/D48555