[PKT_SCHED]: Fix dsmark to apply changes consistent

Fixes dsmark to do all configuration sanity checks first and
only apply the changes if all of them can be applied without
any errors. Also fixes the weak sanity checks for DSMARK_VALUE
and DSMASK_MASK.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>