block: adjust formatting for large minors and add ext_range sysfs attr

With extended minors and the soon-to-follow debug feature, large minor
numbers for block devices will be common.  This patch does the
followings to make printouts pretty.

* Adapt print formats such that large minors don't break the
  formatting.

* For extended MAJ:MIN, %02x%02x for MAJ:MIN used in
  printk_all_partitions() doesn't cut it anymore.  Update it such that
  %03x:%05x is used if either MAJ or MIN doesn't fit in %02x.

* Implement ext_range sysfs attribute which shows total minors the
  device can use including both conventional minor space and the
  extended one.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>