block: do not initialise globals to 0 or NULL

This patch fixes the checkpatch.pl error to blk-exec.c:

ERROR: do not initialise globals to 0 or NULL
Signed-off-by: Wei Tang <tangwei@cmss.chinamobile.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-core.c

@@ -51,7 +51,7 @@ DEFINE_IDA(blk_queue_ida);
 /*
  * For the allocated request tables
  */
-struct kmem_cache *request_cachep = NULL;
+struct kmem_cache *request_cachep;
 
 /*
  * For queue allocation