ipv6: Allow ipv4 wildcard binds after ipv6 address binds

The IPv4 wildcard (0.0.0.0) address does not intersect
in any way with explicit IPv6 addresses.  These two should
be permitted, but the IPv4 conflict code checks the ipv6only
bit as part of the test.  Since binding to an explicit IPv6
address restricts the socket to only that IPv6 address, the
side-effect is that the socket behaves as v6-only.  By
explicitely setting ipv6only in this case, allows the 2 binds
to succeed.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>