Reduce number of DNS queries in mountd.

As reported by a FreeNAS user (see https://redmine.ixsystems.com/issues/55728),
mountd does more calls to getnameinfo() than it needs to; this changes it to
only call it for the RPC calls it needs the name information for.

Reported by: Dave Flowers
Reviewed by: imp, mav
Approved by: mav (mentor)
MFC after: 2 weeks
Sponsored by: iXsystems Inc
Differential Revision: https://reviews.freebsd.org/D18430