net: ipmr: always define mroute_reg_vif_num

Before mroute_reg_vif_num was defined only if any of the CONFIG_PIMSM_
options were set, but that's not really necessary as the size of the
struct is the same in both cases (checked with pahole, both cases size
is 3256 bytes) and we can remove some unnecessary ifdefs to simplify the
code.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>