bcachefs: Preallocate trans mem in bch2_migrate_index_update()

This will help avoid transaction restarts.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/move.c

@@ -67,7 +67,7 @@ static int bch2_migrate_index_update(struct bch_write_op *op)
 	bch2_bkey_buf_init(&_insert);
 	bch2_bkey_buf_realloc(&_insert, c, U8_MAX);
 
-	bch2_trans_init(&trans, c, BTREE_ITER_MAX, 0);
+	bch2_trans_init(&trans, c, BTREE_ITER_MAX, 1024);
 
 	iter = bch2_trans_get_iter(&trans, m->btree_id,
 				   bkey_start_pos(&bch2_keylist_front(keys)->k),