blktrace: mark ddir_act[] const

Impact: cleanup

ddir_act and what2act always stay immutable.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <49C89415.5080503@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>