ovl: use is_subdir() for testing if one thing is a subdir of another

ovl: use is_subdir() for testing if one thing is a subdir of another

Rather than using lock_rename(), use the more obvious is_subdir() for
ensuring that neither upper nor workdir contain the other.
Also be explicit in the comment that the two directories cannot be the
same.

As this is a point-it-time sanity check and does not provide any
on-going guarantees, the removal of locking does not introduce any
interesting races.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20260224222542.3458677-14-neilb@ownmail.net
Signed-off-by: Christian Brauner <brauner@kernel.org>