Remove /zvol/ path component for zvol devices

As part of commit f162433deb the /zvol/
path component was added for zvol devices.  This ensured all zvol
devices would be created by udev in /dev/zvol/<pool>/<dataset>, as
opposed to the previous /dev/<pool>/<dataset> path.  Logically, it
was nice to organize them in a directory much like Solaris does.

However, while initial testing showed this to work fine with modern
kernels it does not appear to be supported under RHEL5.  The extra
path component triggers a NULL deref in create_dir().  Anyway, to
avoid having different zvol path names based on your kernel version
its more consistent simply to revert to the original naming convention.
If you really want the zvol component you can always add custom
udev rules to do exactly this.

We can revisiting this change again once we are willing to drop
support for RHEL5 and similar older distros.

module/zcommon/include/sys/fs/zfs.h

@@ -691,7 +691,7 @@ typedef struct ddt_histogram {
 #define	ZFS_DEV		"/dev/zfs"
 
 /* general zvol path */
-#define	ZVOL_DIR	"/dev/zvol"
+#define	ZVOL_DIR	"/dev"
 
 #define	ZVOL_MAJOR		230
 #define	ZVOL_MINOR_BITS		4

module/zfs/vdev.c

@@ -1072,6 +1072,15 @@ vdev_open_child(void *arg)
 boolean_t
 vdev_uses_zvols(vdev_t *vd)
 {
+/*
+ * Stacking zpools on top of zvols is unsupported until we implement a method
+ * for determining if an arbitrary block device is a zvol without using the
+ * path.  Solaris would check the 'zvol' path component but this does not
+ * exist in the Linux port, so we really should do something like stat the
+ * file and check the major number.  This is complicated by the fact that
+ * we need to do this portably in user or kernel space.
+ */
+#if 0
 	int c;
 
 	if (vd->vdev_path && strncmp(vd->vdev_path, ZVOL_DIR,
@@ -1080,6 +1089,7 @@ vdev_uses_zvols(vdev_t *vd)
 	for (c = 0; c < vd->vdev_children; c++)
 		if (vdev_uses_zvols(vd->vdev_child[c]))
 			return (B_TRUE);
+#endif
 	return (B_FALSE);
 }
 

module/zfs/zvol.c

@@ -1066,7 +1066,7 @@ zvol_alloc(dev_t dev, const char *name)
 	zv->zv_disk->fops = &zvol_ops;
 	zv->zv_disk->private_data = zv;
 	zv->zv_disk->queue = zv->zv_queue;
-	snprintf(zv->zv_disk->disk_name, DISK_NAME_LEN, "zvol/%s", name);
+	snprintf(zv->zv_disk->disk_name, DISK_NAME_LEN, "%s", name);
 
 	return zv;