ipv4: Unmask upper DSCP bits in ip_sock_rt_tos()

The function is used to read the DS field that was stored in IPv4
sockets via the IP_TOS socket option so that it could be used to
initialize the flowi4_tos field before resolving an output route.

Unmask the upper DSCP bits so that in the future the output route lookup
could be performed according to the full DSCP value.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>