Move ZVOL_DIR back to zfs.h

This was previously moved because nothing else in-tree uses it, but
evidently DilOS uses it out of tree.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Ryan Moeller <freqlabs@freebsd.org>
Closes #10361 
Closes #10685

include/sys/fs/zfs.h

@@ -1173,6 +1173,9 @@ typedef struct ddt_histogram {
 
 #define	ZFS_SUPER_MAGIC	0x2fc12fc1
 
+/* general zvol path */
+#define	ZVOL_DIR		"/dev/zvol/"
+
 #define	ZVOL_MAJOR		230
 #define	ZVOL_MINOR_BITS		4
 #define	ZVOL_MINOR_MASK		((1U << ZVOL_MINOR_BITS) - 1)