block: Fix dev_t minor allocation lifetime

commit 2da78092dda13f1efd26edbbf99a567776913750 upstream.

Releases the dev_t minor when all references are closed to prevent
another device from acquiring the same major/minor.

Since the partition's release may be invoked from call_rcu's soft-irq
context, the ext_dev_idr's mutex had to be replaced with a spinlock so
as not so sleep.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
[bwh: Backported to 3.2:
 - Adjust filename
 - idr insertion API is different, and blk_alloc_devt() is preallocating
   a node in a different way]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>