block: disable preemption before using sched_clock()

Commit 9195291e added calls to
sched_clock() from preemptible code. sched_clock() is both the
wrong interface AND cannot be called without preempt disabled.

Apply a temporary fix to get rid of the warnings, a real patch
is in the works.
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

include/linux/blkdev.h

@@ -1211,14 +1211,23 @@ struct work_struct;
 int kblockd_schedule_work(struct request_queue *q, struct work_struct *work);
 
 #ifdef CONFIG_BLK_CGROUP
+/*
+ * This should not be using sched_clock(). A real patch is in progress
+ * to fix this up, until that is in place we need to disable preemption
+ * around sched_clock() in this function and set_io_start_time_ns().
+ */
 static inline void set_start_time_ns(struct request *req)
 {
+	preempt_disable();
 	req->start_time_ns = sched_clock();
+	preempt_enable();
 }
 
 static inline void set_io_start_time_ns(struct request *req)
 {
+	preempt_disable();
 	req->io_start_time_ns = sched_clock();
+	preempt_enable();
 }
 
 static inline uint64_t rq_start_time_ns(struct request *req)