net/ipv6: allow any source address for sendmsg pktinfo with ip_nonlocal_bind

When freebind feature is set of an IPv6 socket, any source address can
be used when sending UDP datagrams using IPv6 PKTINFO ancillary
message. Global non-local bind feature was added in commit
35a256fe

 ("ipv6: Nonlocal bind") for IPv6. This commit also allows
IPv6 source address spoofing when non-local bind feature is enabled.

Signed-off-by: Vincent Bernat <vincent@bernat.im>
Signed-off-by: David S. Miller <davem@davemloft.net>