nfscl: Add NFSD_CURVNET macros to nfsclient syscall

Although the nfsclient syscall is used for client side,
it does set up server side krpc for callbacks. As such,
it needs to have the vnet set. This patch does this.
Without this patch, the system would crash when the
nfscbd(8) daemon was killed.

Reported by: freebsd@walstatt-de.de
MFC after: 3 months