erofs-utils: mkfs: fix `-Eall-fragments` for multi-threaded compression

erofs-utils: mkfs: fix `-Eall-fragments` for multi-threaded compression

`may_packing` in __z_erofs_compress_one() is still bypassed when
multi-threaded compression is enabled, which is unexpected.

Furthermore, multi-threaded `-Eall-fragments,ztailpacking` can
sometimes corrupt images.  Let's fix it.

Fixes: 882ad1c3157f ("erofs-utils: mkfs: fix `-Eall-fragments` for multi-threaded compression")
Fixes: 10c1590c0920 ("erofs-utils: enable multi-threaded support for `-Eall-fragments`")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250102024225.2433419-1-hsiangkao@linux.alibaba.com