[PKT_SCHED]: GRED: Use a central table definition change procedure

Introduces a function gred_change_table_def() acting as a central
point to change the table definition.

Adds missing validations for table definition: MAX_DPs > DPs > 0
and def_DP < DPs thus fixing possible invalid memory reference
oopses. Only root could do it but having a typo crashing the
machine is a bit hard.

Adds missing locking while changing the table definition, the
operation of changing the number of DPs and removing shadowed VQs
may not be interrupted by a dequeue.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>