lightnvm: pblk: stop taking the free lock in in pblk_lines_free

pblk_line_meta_free might sleep (it can end up calling vfree, depending
on how we allocate lba lists), and this can lead to a BUG()
if we wake up on a different cpu and release the lock.

As there is no point of grabbing the free lock when pblk has shut down,
remove the lock.

Signed-off-by: Hans Holmberg <hans.holmberg@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/lightnvm/pblk-init.c

@@ -584,14 +584,12 @@ static void pblk_lines_free(struct pblk *pblk)
 	struct pblk_line *line;
 	int i;
 
-	spin_lock(&l_mg->free_lock);
 	for (i = 0; i < l_mg->nr_lines; i++) {
 		line = &pblk->lines[i];
 
 		pblk_line_free(line);
 		pblk_line_meta_free(l_mg, line);
 	}
-	spin_unlock(&l_mg->free_lock);
 
 	pblk_line_mg_free(pblk);