ipv6: bool conversions phase1

ipv6_opt_accepted() returns a bool, and can use const pointers

ipv6_addr_equal(), ipv6_addr_any(), ipv6_addr_loopback(),
ipv6_addr_orchid() return a bool.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>