bcachefs: Dirent repair code

bcachefs: Dirent repair code

There was a bug for awhile in previous kernels where we weren't
computing dirent name lengths correctly and we weren't zeroing out
padding at the end of dirents (due to struct bch_dirent changing size by
adding __attribute__((aligned)), and not updating other code to use
offsetof).

This patch fixes dirents with junk at the end, by going off of the
dirent's hash.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>