Push down td in nfsrvd_dorpc() - make it use curthread instead
of it being explicitly passed as an argument. No functional changes.

The big picture here is that I want to get rid of the 'td' argument
being passed everywhere, and this is the first piece that affects
the NFS server.

Reviewed by: rmacklem
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D19417