net: vrf: correct FRA_L3MDEV encode type

FRA_L3MDEV is defined as U8, but is being added as a U32 attribute. On
big endian architecture, this results in the l3mdev entry not being
added to the FIB rules.

Fixes: 1aa6c4f6 ("net: vrf: Add l3mdev rules on first device create")
Signed-off-by: Jeff Barnhill <0xeffeff@gmail.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>