certs: make blacklisted hash available in klog

certs: make blacklisted hash available in klog

One common situation triggering this log statement are duplicate hashes
reported by the system firmware.

These duplicates should be removed from the firmware.

Without logging the blacklisted hash triggering the issue however the users
can not report it properly to the firmware vendors and the firmware vendors
can not easily see which specific hash is duplicated.

While changing the log message also use the dedicated ERR_PTR format
placeholder for the returned error value.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>