bcachefs: Fix bch2_gc_btree_gens()

Since we're using a NOT_EXTENTS iterator, we shouldn't be setting the
iter pos to the start of the extent.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_gc.c

@@ -1197,8 +1197,6 @@ static int bch2_gc_btree_gens(struct bch_fs *c, enum btree_id btree_id)
 			bch2_bkey_buf_reassemble(&sk, c, k);
 			bch2_extent_normalize(c, bkey_i_to_s(sk.k));
 
-			bch2_btree_iter_set_pos(iter, bkey_start_pos(&sk.k->k));
-
 			bch2_trans_update(&trans, iter, sk.k, 0);
 
 			ret = bch2_trans_commit(&trans, NULL, NULL,