lightnvm: pblk: fix lock order in pblk_rb_tear_down_check

In pblk_rb_tear_down_check() the spinlock functions are not
called in proper order.

Fixes: a4bd217b ("lightnvm: physical block device (pblk) target")
Signed-off-by: Igor Konopko <igor.j.konopko@intel.com>
Reviewed-by: Javier González <javier@javigon.com>
Reviewed-by: Hans Holmberg <hans.holmberg@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>