ip6mr: limit IPv6 MRT_TABLE identifiers

We did this for IPv4 in b49d3c1e "net: ipmr: limit MRT_TABLE
identifiers" but we need to do it for IPv6 as well.  On IPv6 the name
is "pim6reg" instead of "pimreg" so there is one less digit allowed.

The strcpy() is in ip6mr_reg_vif().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>