lightnvm: pblk: add lock protection to list operations

Protect the list_add on the pblk_line_init_bb() error
path in case this code is used for some other purpose
in the future.

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