When a line allocation fails, for example, due to having too many bad
blocks, free its metadata correctly.

Fixes: a4bd217b "lightnvm: physical block device (pblk) target"
Signed-off-by: NJavier González <javier@cnexlabs.com>
Reviewed-by: NMatias Bjørling <matias@cnexlabs.com>
Signed-off-by: NJens Axboe <axboe@fb.com>