Use udev to create /dev/zvol/[dataset_name] links

This commit allows zvols with names longer than 32 characters, which
fixes issue on https://github.com/behlendorf/zfs/issues/#issue/102.

Changes include:
- use /dev/zd* device names for zvol, where * is the device minor
  (include/sys/fs/zfs.h, module/zfs/zvol.c).
- add BLKZNAME ioctl to get dataset name from userland
  (include/sys/fs/zfs.h, module/zfs/zvol.c, cmd/zvol_id).
- add udev rule to create /dev/zvol/[dataset_name] and the legacy
  /dev/[dataset_name] symlink. For partitions on zvol, it will create
  /dev/zvol/[dataset_name]-part* (etc/udev/rules.d/60-zvol.rules,
  cmd/zvol_id).

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

include/sys/fs/zfs.h

@@ -707,6 +707,7 @@ typedef struct ddt_histogram {
 #define	ZVOL_MINOR_BITS		4
 #define	ZVOL_MINOR_MASK		((1U << ZVOL_MINOR_BITS) - 1)
 #define	ZVOL_MINORS		(1 << 4)
+#define	ZVOL_DEV_NAME		"zd"
 
 #define	ZVOL_PROP_NAME		"name"
 #define	ZVOL_DEFAULT_BLOCKSIZE	8192
@@ -781,6 +782,11 @@ typedef enum zfs_ioc {
 	ZFS_IOC_EVENTS_CLEAR,
 } zfs_ioc_t;
 
+/*
+ * zvol ioctl to get dataset name
+ */
+#define BLKZNAME		_IO(0x12,125)
+
 /*
  * Internal SPA load state.  Used by FMA diagnosis engine.
  */