nfsd: fix krb5 handling of anonymous principals

krb5 mounts started failing as of
683428fa "sunrpc: Update svcgss xdr
handle to rpsec_contect cache".

The problem is that mounts are usually done with some host principal
which isn't normally mapped to any user, in which case svcgssd passes
down uid -1, which the kernel is then expected to map to the
export-specific anonymous uid or gid.

The new uid_valid/gid_valid checks were therefore causing that downcall
to fail.

(Note the regression may not have been seen with older userspace that
tended to map unknown principals to an anonymous id on their own rather
than leaving it to the kernel.)

Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>