erofs: fix long xattr name prefix placement

Currently, xattr name prefixes are forcibly placed into the packed
inode if the fragments feature is enabled, and users have no option
to put them in plain form directly on disk.

This is inflexible. First, as mentioned above, users should be able
to store unwrapped long xattr name prefixes unconditionally
(COMPAT_PLAIN_XATTR_PFX). Second, since we now have the new metabox
inode to store metadata, it should be used when available instead
of the packed inode.

Fixes: 41409132 ("erofs: implement metadata compression")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>