block: add REQ_DRV bit

Set aside a bit in the request/bio flags for driver use.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NSagi Grimberg <sagi@grimberg.me>
Reviewed-by: NHannes Reinecke <hare@suse.com>
Reviewed-by: NJohannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

include/linux/blk_types.h

@@ -229,6 +229,9 @@ enum req_flag_bits {
 	/* command specific flags for REQ_OP_WRITE_ZEROES: */
 	__REQ_NOUNMAP,		/* do not free blocks when zeroing */
 
+	/* for driver use */
+	__REQ_DRV,
+
 	__REQ_NR_BITS,		/* stops here */
 };
 
@@ -249,6 +252,8 @@ enum req_flag_bits {
 
 #define REQ_NOUNMAP		(1ULL << __REQ_NOUNMAP)
 
+#define REQ_DRV			(1ULL << __REQ_DRV)
+
 #define REQ_FAILFAST_MASK \
 	(REQ_FAILFAST_DEV | REQ_FAILFAST_TRANSPORT | REQ_FAILFAST_DRIVER)