Move bi_size and bi_sectors into the non-varint portion of the inode, so
that the write path can update them without going through the relatively
expensive unpack/pack operations.

Other changes:
 - Add a field for the offset of the varint section, so we can add new
   non-varint fields without needing a new inode type, like alloc_v3
 - Move bi_mode into the flags field, so that the varint section can be
   u64 aligned
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>