Unmask the upper DSCP bits when calling ipmr_fib_lookup() so that in the
future it could perform the FIB lookup according to the full DSCP value.

Note that ipmr_fib_lookup() performs a FIB rule lookup (returning the
relevant routing table) and that IPv4 multicast FIB rules do not support
matching on TOS / DSCP. However, it is still worth unmasking the upper
DSCP bits in case support for DSCP matching is ever added.
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/20240821125251.1571445-7-idosch@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>