Use ZFS_DEV macro instead of literals

The rest of the code/comments use ZFS_DEV, so sync with that.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes #8912

lib/libzpool/util.c

@@ -223,7 +223,7 @@ pool_active(void *unused, const char *name, uint64_t guid,
 	 * Use ZFS_IOC_POOL_SYNC to confirm if a pool is active
 	 */
 
-	fd = open("/dev/zfs", O_RDWR);
+	fd = open(ZFS_DEV, O_RDWR);
 	if (fd < 0)
 		return (-1);