ipmr: ip6mr: Create new sockopt to clear mfc cache or vifs

Currently the only way to clear the forwarding cache was to delete the
entries one by one using the MRT_DEL_MFC socket option or to destroy and
recreate the socket.

Create a new socket option which with the use of optional flags can
clear any combination of multicast entries (static or not static) and
multicast vifs (static or not static).

Calling the new socket option MRT_FLUSH with the flags MRT_FLUSH_MFC and
MRT_FLUSH_VIFS will clear all entries and vifs on the socket except for
static entries.

Signed-off-by: Callum Sinclair <callum.sinclair@alliedtelesis.co.nz>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>