block: store elevator state in request

Add an rq private RQF_ELV flag, which tells the block layer that this
request was initialized on a queue that has an IO scheduler attached.
This allows for faster checking in the fast path, rather than having to
deference rq->q later on.

Elevator switching does full quiesce of the queue before detaching an
IO scheduler, so it's safe to cache this in the request itself.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blk-mq.h

@@ -55,6 +55,8 @@ typedef __u32 __bitwise req_flags_t;
 #define RQF_MQ_POLL_SLEPT	((__force req_flags_t)(1 << 20))
 /* ->timeout has been called, don't expire again */
 #define RQF_TIMED_OUT		((__force req_flags_t)(1 << 21))
+/* queue has elevator attached */
+#define RQF_ELV			((__force req_flags_t)(1 << 22))
 
 /* flags that prevent us from merging requests: */
 #define RQF_NOMERGE_FLAGS \