bcache: remove redundant assignment to variable cur_idx

Variable cur_idx is being initialized with a value that is never read,
it is being re-assigned later in a while-loop. Remove the redundant
assignment. Cleans up clang scan build warning:

drivers/md/bcache/writeback.c:916:2: warning: Value stored to 'cur_idx'
is never read [deadcode.DeadStores]
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Coly Li <colyli@suse.de>
Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20231120052503.6122-4-colyli@suse.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/writeback.c

@@ -913,7 +913,7 @@ static int bch_dirty_init_thread(void *arg)
 	int cur_idx, prev_idx, skip_nr;
 
 	k = p = NULL;
-	cur_idx = prev_idx = 0;
+	prev_idx = 0;
 
 	bch_btree_iter_init(&c->root->keys, &iter, NULL);
 	k = bch_btree_iter_next_filter(&iter, &c->root->keys, bch_ptr_bad);