ovl: fix trusted xattr escape prefix matching

ovl: fix trusted xattr escape prefix matching

In the trusted.* xattr namespace, ovl_is_escaped_xattr() compares
one byte less than the escaped overlay xattr prefix length. This makes
it match "trusted.overlay.overlay" without requiring the trailing dot.

As a result, an xattr such as "trusted.overlay.overlayfoo" is
incorrectly treated as an escaped overlay xattr. This can be reproduced
by setting "trusted.overlay.overlayfoo" on a lower file and listing xattrs
through an overlay mount. listxattr() then exposes it as
"trusted.overlay.oo", and a following getxattr() on that listed name fails
with ENODATA.

Compare the full escaped prefix, including the trailing dot, so
similarly-prefixed private xattrs are not misclassified.

Fixes: dad02fad84cbc ("ovl: Support escaped overlay.* xattrs")
Signed-off-by: Yichong Chen <chenyichong@uniontech.com>
Link: https://patch.msgid.link/20260708082221.633602-1-chenyichong@uniontech.com
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>