[IPV6]: fix section mismatch warnings

Removed useless and buggy __exit section in the different
ipv6 subsystems. Otherwise they will be called inside an
init section during rollbacking in case of an error in the
protocol initialization.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>