bcachefs: Delete warning from promote_alloc()

It's possible to see a -BCH_ERR_ENOSPC_disk_reservation here, and that's
fine.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/io.c

@@ -2057,10 +2057,11 @@ static struct promote_op *__promote_alloc(struct btree_trans *trans,
 				.write_flags	= BCH_WRITE_ALLOC_NOWAIT|BCH_WRITE_CACHED,
 			},
 			btree_id, k);
+	/*
+	 * possible errors: -BCH_ERR_nocow_lock_blocked,
+	 * -BCH_ERR_ENOSPC_disk_reservation:
+	 */
 	if (ret) {
-		WARN_ONCE(ret != -BCH_ERR_nocow_lock_blocked,
-			  "%s: saw unknown error %s\n", __func__, bch2_err_str(ret));
-
 		ret = rhashtable_remove_fast(&c->promote_table, &op->hash,
 					bch_promote_params);
 		BUG_ON(ret);