nfs_commonsubs.c: Make all upper case user domain work

Without this patch, an all upper case user domain name
(as specified by nfsuserd(8)) would not work.
I believe this was done so that Kerberos realms were
not confused with user domains.

Now, RFC8881 specifies that the user domain name is a
DNS name. As such, all upper case names should work.

This patch fixes this case so that it works. The custom
comparison function is no longer needed.

PR: 282620

(cherry picked from commit 0347ddf41f4226c0351d2d1d78f09e8300ebac93)