net: ipmr: move the tbl id check in ipmr_new_table

Move the table id check in ipmr_new_table and make it return error
pointer. We need this change for the upcoming netlink table manipulation
support in order to avoid code duplication and a race condition.

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