bcachefs: Fix compat path for old inode formats

bcachefs: Fix compat path for old inode formats

Old inode formats don't have all the fields of the current inode format:
when unpacking inodes in the current format we can thus skip zeroing out
the destination buffer, but that doesn't work on for the old formats.

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