lightnvm: pblk: free metadata on line alloc failure

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: Javier González <javier@cnexlabs.com>
Reviewed-by: Matias Bjørling <matias@cnexlabs.com>
Signed-off-by: Jens Axboe <axboe@fb.com>