block: Fix dev_t minor allocation lifetime

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>
[lizf: Backported to 3.4:
 - adjust context
 - remove idr_preload() and idr_preload_end()]
Signed-off-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>