lightnvm: pblk: recheck for bad lines at runtime

Bad blocks can grow at runtime. Check that the number of valid blocks in
a line are within the sanity threshold before allocating the line for
new writes.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>