ipv4: Prepare ip_route_output() to future .flowi4_tos conversion.

ipv4: Prepare ip_route_output() to future .flowi4_tos conversion.

Convert the "tos" parameter of ip_route_output() to dscp_t. This way
we'll have a dscp_t value directly available when .flowi4_tos will
eventually be converted to dscp_t.

All ip_route_output() callers but one set this "tos" parameter to 0 and
therefore don't need to be adapted to the new prototype.

Only br_nf_pre_routing_finish() needs conversion. It can just use
ip4h_dscp() to get the DSCP field from the IPv4 header.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/0f10d031dd44c70aae9bc6e19391cb30d5c2fe71.1730928699.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>