bcachefs: Split out btree_write_submit_wq

Split the workqueues for btree read completions and btree write
submissions; we don't want concurrency control on btree read
completions, but we do want concurrency control on write submissions,
else blocking in submit_bio() will cause a ton of kworkers to be
allocated.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
diff --git a/fs/bcachefs/bcachefs.h b/fs/bcachefs/bcachefs.h
index 2a538eb..2992a64 100644
--- a/fs/bcachefs/bcachefs.h
+++ b/fs/bcachefs/bcachefs.h
@@ -790,7 +790,8 @@ struct bch_fs {
 
 	/* BTREE CACHE */
 	struct bio_set		btree_bio;
-	struct workqueue_struct	*io_complete_wq;
+	struct workqueue_struct	*btree_read_complete_wq;
+	struct workqueue_struct	*btree_write_submit_wq;
 
 	struct btree_root	btree_roots_known[BTREE_ID_NR];
 	DARRAY(struct btree_root) btree_roots_extra;