yaffs2: convert user_namespace to mnt_idmap

commit c1632a0f [fs: port ->setattr() to
pass mnt_idmap] changes the parameters to attr operations to a
mnt_idmap.

yaffs2 isn't using the user namespace directly, so we switch to the
default map.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>