tree: do not issue an error when subsys lookup fails during scanning

tree: do not issue an error when subsys lookup fails during scanning

The scan operation is not atomically done and the sysfs might change
while we are iterating over it. Thus, it's possible that we find a
controller but when we try to lookup the corresponding subsystem it might
already destroyed and removed.

This error makes blktests fail because it finds controllers controller
which are not under control of blktests, instead they are created and
destroyed by the udev auto connect rules.

These resources appear and disappear while the test runs but when we
scan sysfs we issue errors for unrelated resources. Thus just do not
issue a error, turn this into debug log message.

Anyway, we already do just return error codes for other reason in this
function anyway without logging.

Signed-off-by: Daniel Wagner <dwagner@suse.de>