bcachefs: iter->real_pos

We need to differentiate between the search position of a btree
iterator, vs. what it actually points at (what we found). This matters
for extents, where iter->pos will typically be the start of the key we
found and iter->real_pos will be the end of the key we found (which soon
won't necessarily be in the same btree node!) and it will also matter
for snapshots.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_types.h

@@ -247,6 +247,8 @@ enum btree_iter_uptodate {
 struct btree_iter {
 	struct btree_trans	*trans;
 	struct bpos		pos;
+	/* what we're searching for/what the iterator actually points to: */
+	struct bpos		real_pos;
 	struct bpos		pos_after_commit;
 
 	u16			flags;

fs/bcachefs/btree_update_leaf.c

@@ -219,7 +219,7 @@ static inline void btree_insert_entry_checks(struct btree_trans *trans,
 {
 	struct bch_fs *c = trans->c;
 
-	BUG_ON(bkey_cmp(insert->k.p, iter->pos));
+	BUG_ON(bkey_cmp(insert->k.p, iter->real_pos));
 	BUG_ON(bch2_debug_check_bkeys &&
 	       bch2_bkey_invalid(c, bkey_i_to_s_c(insert),
 				 __btree_node_type(iter->level, iter->btree_id)));