ipv4: Convert __fib_validate_source() to dscp_t.

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.

Only fib_validate_source() actually calls __fib_validate_source().
Since it already has a dscp_t variable to pass as parameter, we only
need to remove the inet_dscp_to_dsfield() conversion.

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/8206b0a64a21a208ed94774e261a251c8d7bc251.1728302212.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>