nfs: label NFSv4 CREATE symlink arguments more helpfully

Currently it's hard to tell which field is the name of the symlink and
which is the target; both are labeled just "Name".

Ping-Bug: 10649
Change-Id: I1322a3652ffbcee920a6808dcc5613cefe8d8588
Reviewed-on: https://code.wireshark.org/review/5142
Reviewed-by: Michael Mann <mmann78@netscape.net>