gtp: Prepare ip4_route_output_gtp() to .flowi4_tos conversion.

gtp: Prepare ip4_route_output_gtp() to .flowi4_tos conversion.

Use inet_sk_dscp() to get the socket DSCP value as dscp_t, instead of
ip_sock_rt_tos() which returns a __u8. This will ease the conversion
of fl4->flowi4_tos to dscp_t, which now just becomes a matter of
dropping the inet_dscp_to_dsfield() call.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/06bdb310a075355ff059cd32da2efc29a63981c9.1737034675.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>