ipmr: Fix ptrdiff_t print formatting

ipmr_vif_seq_show() prints the difference between two pointers with the
format string %2zd (z for size_t), however the correct format string is
%2td instead (t for ptrdiff_t).

The same bug in ip6mr_vif_seq_show() was already fixed long ago by
commit d430a227 ("bogus format in ip6mr").

Signed-off-by: James Hogan <jhogan@kernel.org>
Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>