NLM: Rewrite IPv4 privileged requester's check

Clean up.

For consistency, rewrite the IPv4 check to match the same style as the
new IPv6 check.  Note that ipv4_is_loopback() is somewhat broader in
its interpretation of what is a loopback address than simply
"127.0.0.1".

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>