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: NKeith Busch <keith.busch@intel.com>
Cc: stable@kernel.org
Signed-off-by: NJens Axboe <axboe@fb.com>