xen/blkfront: use bdget_disk

info->dev is never initialized to anything, so bdget(info->dev) is
meaningless.  Get rid of info->dev, and use bdget_disk on the gendisk.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>