ipv4: udp: Unmask upper DSCP bits during early demux

Unmask the upper DSCP bits when performing source validation for
multicast packets during early demux. In the future, this will allow us
to perform the FIB lookup which is performed as part of source
validation according to the full DSCP value.

No functional changes intended since the upper DSCP bits are masked when
comparing against the TOS selectors in FIB rules and routes.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Acked-by: Florian Westphal <fw@strlen.de>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20240821125251.1571445-12-idosch@nvidia.com


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