ipmr: fix tables suspicious RCU usage

Similar to the previous patch, plumb the RCU lock inside
the ipmr_get_table(), provided a lockless variant and apply
the latter in the few spots were the lock is already held.

Fixes: 709b46e8 ("net: Add compat ioctl support for the ipv4 multicast ioctl SIOCGETSGCNT")
Fixes: f0ad0860 ("ipv4: ipmr: support multiple tables")
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>