truss: Properly display first argument to nmount

The first argument to nmount(2) is an nvlist in the form of an iovec,
which truss already knows how to decode. Set the correct flag so
this happens automatically.

MFC after: 1 week
PR: 290667

(cherry picked from commit b9f848ecbafce4e56ba9c8b7993b85347e83484a)