LU-14477 lnet: handle possiblity of IPv6 being unavailable.

LU-14477 lnet: handle possiblity of IPv6 being unavailable.

If CONFIG_IPV6 is not enabled, the attempt to create an IPv6 socket
for accepting new incoming connections will fail.  In that case
we need to creae an IPv4 socket instead.

Also ipv6_dev_get_saddr will not be available, so we must not include
the code that tries to use it.

Test-Parameters: trivial
Fixes: e4fa181abf10 ("LU-10391 lnet: allow creation of IPv6 socket")
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ib576c7ea498c90f549958f3c1aa0beb7fe2b66ad
Reviewed-on: https://review.whamcloud.com/41791
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>