zvol: Implement zvol threading as a Property

Currently, zvol threading can be switched through the zvol_request_sync
module parameter system-wide. By making it a zvol property, zvol
threading can be switched per zvol.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #15409

include/sys/fs/zfs.h

@@ -192,6 +192,7 @@ typedef enum {
 	ZFS_PROP_REDACT_SNAPS,
 	ZFS_PROP_SNAPSHOTS_CHANGED,
 	ZFS_PROP_PREFETCH,
+	ZFS_PROP_VOLTHREADING,
 	ZFS_NUM_PROPS
 } zfs_prop_t;