zvol_os: fix build on Linux <3.13

99741bd introduced zvol_num_taskqs, but put it behind the HAVE_BLK_MQ
define, preventing builds on versions of Linux that don't have it
(<3.13, incl EL7).

Nothing about it seems dependent on blk-mq, so this just moves it out
from behind that define and so fixes the build.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>

module/os/linux/zfs/zvol_os.c

@@ -81,9 +81,10 @@ static boolean_t zvol_use_blk_mq = B_FALSE;
  * read and write tests to a zvol in an NVMe pool (with 16 CPUs).
  */
 static unsigned int zvol_blk_mq_blocks_per_thread = 8;
-static unsigned int zvol_num_taskqs = 0;
 #endif
 
+static unsigned int zvol_num_taskqs = 0;
+
 #ifndef	BLKDEV_DEFAULT_RQ
 /* BLKDEV_MAX_RQ was renamed to BLKDEV_DEFAULT_RQ in the 5.16 kernel */
 #define	BLKDEV_DEFAULT_RQ BLKDEV_MAX_RQ