rtnetlink: Mask the rta_type when range checking

Range/validity checks on rta_type in rtnetlink_rcv_msg() do
not account for flags that may be set.  This causes the function
to return -EINVAL when flags are set on the type (for example
NLA_F_NESTED).

Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>