selftest: net: Add basic functionality tests for ipmr.

selftest: net: Add basic functionality tests for ipmr.

The new test exercise paths, where RTNL is needed, to
catch lockdep splat:

  setsockopt
    MRT_INIT / MRT_DONE
    MRT_ADD_VIF / MRT_DEL_VIF
    MRT_ADD_MFC / MRT_DEL_MFC / MRT_ADD_MFC_PROXY / MRT_DEL_MFC_PROXY
    MRT_TABLE
    MRT_FLUSH

  rtnetlink
    RTM_NEWROUTE
    RTM_DELROUTE

  NETDEV_UNREGISTER

I will extend this to cover IPv6 setsockopt() later.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260228221800.1082070-2-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>