blktrace_api.h: fix comment for struct blk_user_trace_setup

'struct blk_user_trace_setup' is passed to BLKTRACESETUP, not
BLKTRACESTART.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/uapi/linux/blktrace_api.h

@@ -131,7 +131,7 @@ enum {
 #define BLKTRACE_BDEV_SIZE	32
 
 /*
- * User setup structure passed with BLKTRACESTART
+ * User setup structure passed with BLKTRACESETUP
  */
 struct blk_user_trace_setup {
 	char name[BLKTRACE_BDEV_SIZE];	/* output */