ipv4: Convert icmp_route_lookup() to dscp_t.

ipv4: Convert icmp_route_lookup() to dscp_t.

Pass a dscp_t variable to icmp_route_lookup(), instead of a plain u8,
to prevent accidental setting of ECN bits in ->flowi4_tos. Rename that
variable ("tos" -> "dscp") to make the intent clear.

While there, reorganise the function parameters to fill up horizontal
space.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/294fead85c6035bcdc5fcf9a6bb4ce8798c45ba1.1727807926.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>