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

If `-Eall-fragments` is specified when multi-threaded compression is
enabled, it should only apply to the packed inode instead of other
real inodes for now.

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/20241012035213.3729725-1-hsiangkao@linux.alibaba.com