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

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>