SUNRPC: Bury "#ifdef IPV6" in svc_create_xprt()

Clean up:  Bruce observed we have more or less common logic in each of
svc_create_xprt()'s callers:  the check to create an IPv6 RPC listener
socket only if CONFIG_IPV6 is set.  I'm about to add another case
that does just the same.

If we move the ifdefs into __svc_xpo_create(), then svc_create_xprt()
call sites can get rid of the "#ifdef" ugliness, and can use the same
logic with or without IPv6 support available in the kernel.

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