lightnvm: refactor spin_unlock in gennvm_get_blk

The spin_unlock is duplicated multiple times. Jump to a single unlock
to improve the code flow.

Signed-off-by: Wenwei Tao <ww.tao0320@gmail.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>