SUNRPC: Use conventional switch statement when reclassifying sockets

Clean up.

Defensive coding: If "family" is ever something that is neither
AF_INET nor AF_INET6, xs_reclassify_socket6() is not the appropriate
default action.  Choose to do nothing in that case.

Introduced by commit 6bc9638a.

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