net: ipmr: make ip_mroute_getsockopt more understandable

Use a switch to determine if optname is correct and set val accordingly.
This produces a much more straight-forward and readable code.

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