overlay: filter out xattr starts with "trusted.overlay."

overlay: filter out xattr starts with "trusted.overlay."

Overlayfs should only filter out xattr starting with
"trusted.overlay.", not "trusted.overlay".

Setting xattrs like "trusted.overlay.xxx" is not allowed.
Setting xattrs like "trusted.overlayxxx" is allowed.

v4.8-rc1 introduced a regression that we can't set xattrs
like "trusted.overlayxxx".  Kernel commit below fixed it
in v4.8:
  fe2b75952347 ovl: Fix OVL_XATTR_PREFIX

This case tests both get/set of these 2 kinds of xattrs.

Pattern "trusted.overlay.xxx" should fail, however the
errno returned by set/get varies among kernel versions.

Pattern "trusted.overlayxxx" shold always work.

CC: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Xiong Zhou <xzhou@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>