bcachefs: Silence gcc warnings about arm arch ABI drift

32-bit arm builds emit a lot of spam like this:

    fs/bcachefs/backpointers.c: In function ‘extent_matches_bp’:
    fs/bcachefs/backpointers.c:15:13: note: parameter passing for argument of type ‘struct bch_backpointer’ changed in GCC 9.1

Apply the change from commit ebcc5928

 ("arm64: Silence gcc warnings
about arch ABI drift") to fs/bcachefs/ to silence them.

Signed-off-by: Calvin Owens <jcalvinowens@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>