ipv4: ipmr: fix NULL pointer deref during unres queue destruction

Fix an oversight in ipmr_destroy_unres() - the net pointer is
unconditionally initialized to NULL, resulting in a NULL pointer
dereference later on.

Fix by adding a net pointer to struct mr_table and using it in
ipmr_destroy_unres().

Signed-off-by: Patrick McHardy <kaber@trash.net>