nfs: NFSv4.2 SEEK, ALLOCATE, DEALLOCATE support

NFSv4.2 is still a draft, but these three ops are implemented at least
for Linux now and are very unlikely to change any further.

Ping-Bug: 10649
Change-Id: Ia2917de59fb2b0ed7191c9f362b7eb60756d7634
Reviewed-on: https://code.wireshark.org/review/5143
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>