ipv6: Remove fallback argument from ip6_hold_safe

net and null_fallback are redundant. Remove null_fallback in favor of
!net check.

Signed-off-by: David Ahern <dsahern@gmail.com>
Acked-by: Wei Wang <weiwan@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>