lightnvm: update closed list outside of intr context

When an I/O finishes, full blocks are moved from the open to the closed
list - a lock is taken to protect the list. This happens at the moment
in the interrupt context, which is not correct.

This patch moves this logic to the block workqueue instead, avoiding
holding a spinlock without interrupt save in an interrupt context.

Signed-off-by: Javier González <javier@cnexlabs.com>
Fixes: ff0e498b ("lightnvm: manage open and closed blocks sepa...")
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>