exportfs: matchhostname() doesn't handle localhost properly

Same change as statd_matchhostname() is necessary for the logic in
exportfs.

Recall that these are "separate but nearly equal" because the exportfs
version requires extra expensive string checking that would be onerous
for statd.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>