When dealing with mounts from user namespaces quota user ids must
be translated relative to s_user_ns, especially when they will be
stored on disk. For purposes of the in-kernel hash table using
init_user_ns is still okay and may help reduce hash collisions,
so continue using init_user_ns there.

These changes introduce the possibility that the conversion
operations in struct qtree_fmt_operations could fail if an id
cannot be legitimately converted. Change these operations to
return int rather than void, and update the callers to handle
failures.
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>