net: ipmr: drop ip_mr_init() mrt_cachep null check as we'll panic if it fails

It's not necessary to check for null as SLAB_PANIC is used and we'll
panic if the alloc fails, so just drop it.

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