Staging: hv: fix sysfs symlink on hv block device

The block device does not create the proper symlink in sysfs because we
forgot to set up the gendisk structure properly.  This patch fixes the
issue.

Signed-off-by: K. Y. Srinivasan <ksrinivasan@novell.com>
Cc: Hank Janssen <hjanssen@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>