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>