net: Fix bug in compat SIOCGETSGCNT handling.

Commit 709b46e8 ("net: Add compat
ioctl support for the ipv4 multicast ioctl SIOCGETSGCNT") added the
correct plumbing to handle SIOCGETSGCNT properly.

However, whilst definiting a proper "struct compat_sioc_sg_req" it
isn't actually used in ipmr_compat_ioctl().

Correct this oversight.

Signed-off-by: David S. Miller <davem@davemloft.net>