[BLOCK] mark some block/ variables cons

the patch below marks various read-only variables in block/* as const,
so that gcc can optimize the use of them; eg gcc will replace the use by
the value directly now and will even remove the memory usage of these.

Signed-off-by: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Jens Axboe <axboe@suse.de>
diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index 97f4e7e..e02c88c 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -1039,7 +1039,7 @@
 
 EXPORT_SYMBOL(blk_queue_invalidate_tags);
 
-static char *rq_flags[] = {
+static const char * const rq_flags[] = {
 	"REQ_RW",
 	"REQ_FAILFAST",
 	"REQ_SORTED",