net: ipmr: ipmr_get_table() returns NULL

The ipmr_get_table() function doesn't return error pointers it returns
NULL on error.

Fixes: 4f75ba69 ("net: ipmr: Add ipmr_rtm_getroute")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>