nfs: clean up fh_expire_type dissection

This is a bit simpler and makes the field searchable.

Ping-Bug: 10649
Change-ID: I68386f987c419c0b6cbc181f29c6bce6c04fcfdb
Reviewed-on: https://code.wireshark.org/review/5145
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>