rpc.tatd: refactor check to see if call is from loopback address

rpc.tatd: refactor check to see if call is from	loopback address

Refactor common logic to check if SM_FOO request is from loopback
address.

We'll have to do something about this for IPv6.  On IPv6-capable
systems, there will be only one AF_INET6 listener.  The loopback caller
will get either an IPv6 loopback address, or a mapped IPv4 loopback --
either way this will be an AF_INET6 address.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Acked-by: Neil Brown <neilb@suse.de>
Signed-off-by: Steve Dickson <steved@redhat.com>