bcachefs: Fix build errors with gcc 10

gcc 10 seems to complain about array bounds in situations where gcc 11
does not - curious.

This unfortunately requires adding some casts for now; we may
investigate getting rid of our __u64 _data[] VLA in a future patch so
that our start[0] members can be VLAs.

Reported-by: John Stoffel <[e-mail]>
Signed-off-by: Kent Overstreet <[e-mail]>

fs/bcachefs/bcachefs_format.h

···
 
 struct bch_sb_field_journal_seq_blacklist {
 	struct bch_sb_field	field;
-
-	struct journal_seq_blacklist_entry start[0];
-	__u64			_data[];
+	struct journal_seq_blacklist_entry start[];
 };
 
 struct bch_sb_field_errors {

fs/bcachefs/btree_trans_commit.c

···
 						       BCH_JSET_ENTRY_overwrite,
 						       i->btree_id, i->level,
 						       i->old_k.u64s);
-				bkey_reassemble(&entry->start[0],
+				bkey_reassemble((struct bkey_i *) entry->start,
 						(struct bkey_s_c) { &i->old_k, i->old_v });
 			}
 
···
 					       BCH_JSET_ENTRY_btree_keys,
 					       i->btree_id, i->level,
 					       i->k->k.u64s);
-			bkey_copy(&entry->start[0], i->k);
+			bkey_copy((struct bkey_i *) entry->start, i->k);
 		}
 
 		trans_for_each_wb_update(trans, wb) {
···
 					       BCH_JSET_ENTRY_btree_keys,
 					       wb->btree, 0,
 					       wb->k.k.u64s);
-			bkey_copy(&entry->start[0], &wb->k);
+			bkey_copy((struct bkey_i *) entry->start, &wb->k);
 		}
 
 		if (trans->journal_seq)

fs/bcachefs/btree_update_interior.c

···
 
 	r->level = entry->level;
 	r->alive = true;
-	bkey_copy(&r->key, &entry->start[0]);
+	bkey_copy(&r->key, (struct bkey_i *) entry->start);
 
 	mutex_unlock(&c->btree_root_lock);
 }

fs/bcachefs/btree_update_interior.h

···
 	struct btree_node_entry *bne = max(write_block(b),
 			(void *) btree_bkey_last(b, bset_tree_last(b)));
 	ssize_t remaining_space =
-		__bch_btree_u64s_remaining(c, b, &bne->keys.start[0]);
+		__bch_btree_u64s_remaining(c, b, bne->keys.start);
 
 	if (unlikely(bset_written(b, bset(b, t)))) {
 		if (remaining_space > (ssize_t) (block_bytes(c) >> 3))

fs/bcachefs/recovery.c

···
 
 		if (entry->u64s) {
 			r->level = entry->level;
-			bkey_copy(&r->key, &entry->start[0]);
+			bkey_copy(&r->key, (struct bkey_i *) entry->start);
 			r->error = 0;
 		} else {
 			r->error = -EIO;