block: Fix dev_t minor allocation lifetime

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 <[email protected]>
Cc: [email protected]
Signed-off-by: Jens Axboe <[email protected]>