xfrm: Unmask upper DSCP bits in xfrm_get_tos()

The function returns a value that is used to initialize 'flowi4_tos'
before being passed to the FIB lookup API in the following call chain:

xfrm_bundle_create()
	tos = xfrm_get_tos(fl, family)
	xfrm_dst_lookup(..., tos, ...)
		__xfrm_dst_lookup(..., tos, ...)
			xfrm4_dst_lookup(..., tos, ...)
				__xfrm4_dst_lookup(..., tos, ...)
					fl4->flowi4_tos = tos
					__ip_route_output_key(net, fl4)

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

Remove IPTOS_RT_MASK since it is no longer used.

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