ipmr: off by one in __ipmr_fill_mroute()

This fixes a smatch warning:
	net/ipv4/ipmr.c +1917 __ipmr_fill_mroute(12) error: buffer overflow
	'(mrt)->vif_table' 32 <= 32

The ipv6 version had the same issue.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>