bcachefs: bpos is misaligned on big endian

bkey embeds a bpos that is misaligned on big endian; this is so that
bch2_bkey_swab() works correctly without having to differentiate between
packed and non-packed keys (a debatable design decision).

This means it can't have the __aligned() tag on big endian.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs_format.h

@@ -151,7 +151,11 @@ struct bpos {
 #else
 #error edit for your odd byteorder.
 #endif
-} __packed __aligned(4);
+} __packed
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+__aligned(4)
+#endif
+;
 
 #define KEY_INODE_MAX			((__u64)~0ULL)
 #define KEY_OFFSET_MAX			((__u64)~0ULL)