bcachefs: Fix array-index-out-of-bounds

We use 0 size arrays as markers, but ubsan doesn't know that - cast them
to a pointer to fix the splat.

Also, make sure this code gets tested a bit more.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bkey.c

@@ -1064,7 +1064,7 @@ void bch2_bkey_swab_key(const struct bkey_format *_f, struct bkey_packed *k)
 {
 	const struct bkey_format *f = bkey_packed(k) ? _f : &bch2_bkey_format_current;
 	u8 *l = k->key_start;
-	u8 *h = (u8 *) (k->_data + f->key_u64s) - 1;
+	u8 *h = (u8 *) ((u64 *) k->_data + f->key_u64s) - 1;
 
 	while (l < h) {
 		swap(*l, *h);

fs/bcachefs/bkey_methods.c

@@ -398,8 +398,12 @@ void __bch2_bkey_compat(unsigned level, enum btree_id btree_id,
 	for (i = 0; i < nr_compat; i++)
 	switch (!write ? i : nr_compat - 1 - i) {
 	case 0:
-		if (big_endian != CPU_BIG_ENDIAN)
+		if (big_endian != CPU_BIG_ENDIAN) {
+			bch2_bkey_swab_key(f, k);
+		} else if (IS_ENABLED(CONFIG_BCACHEFS_DEBUG)) {
 			bch2_bkey_swab_key(f, k);
+			bch2_bkey_swab_key(f, k);
+		}
 		break;
 	case 1:
 		if (version < bcachefs_metadata_version_bkey_renumber)

fs/bcachefs/bkey_methods.h

@@ -129,7 +129,8 @@ static inline void bch2_bkey_compat(unsigned level, enum btree_id btree_id,
 			       struct bkey_packed *k)
 {
 	if (version < bcachefs_metadata_version_current ||
-	    big_endian != CPU_BIG_ENDIAN)
+	    big_endian != CPU_BIG_ENDIAN ||
+	    IS_ENABLED(CONFIG_BCACHEFS_DEBUG))
 		__bch2_bkey_compat(level, btree_id, version,
 				   big_endian, write, f, k);