net/ipv6: Correct PIM6 mrt_lock handling

In the IPv6 multicast routing code the mrt_lock was not being released
correctly in the MFC iterator, as a result adding or deleting a MIF would
cause a hang because the mrt_lock could not be acquired.

This fix is a copy of the code for the IPv4 case and ensures that the lock
is released correctly.

Signed-off-by: Richard Laing <richard.laing@alliedtelesis.co.nz>
Acked-by: Cong Wang <cwang@twopensource.com>
Signed-off-by: David S. Miller <davem@davemloft.net>