ipv4: Convert fib_validate_source() to dscp_t.

Pass a dscp_t variable to fib_validate_source(), instead of a plain u8,
to prevent accidental setting of ECN bits in ->flowi4_tos.

All callers of fib_validate_source() already have a dscp_t variable to
pass as parameter. We just need to remove the inet_dscp_to_dsfield()
conversions.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/08612a4519bc5a3578bb493fbaad82437ebb73dc.1728302212.git.gnault@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>