ipmr: Add reference count to MFC entries

Next commits will introduce MFC notifications through the atomic
fib_notification chain, thus allowing modules to be aware of MFC entries.

Due to the fact that modules may need to hold a reference to an MFC entry,
add reference count to MFC entries to prevent them from being freed while
these modules use them.

The reference counting is done only on resolved MFC entries currently.

Signed-off-by: Yotam Gigi <yotamg@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>