nfsd: don't require low ports for gss requests

In a traditional NFS deployment using auth_unix, the clients are trusted
to correctly report the credentials of their logged-in users.  The
server assumes that only root on client machines is allowed to send
requests from low-numbered ports, so it can use the originating port
number to distinguish "real" NFS clients from NFS clients run by
ordinary users, to prevent ordinary users from spoofing credentials.

The originating port number on a gss-authenticated request is less
important.  The authentication ties the request to a user, and we take
it as proof that that user authorized the request.  The low port number
check no longer adds much.

So, don't enforce low port numbers in the auth_gss case.

Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>