Also note that while 15.10 does change the permissions on /sys/kernel/debug/tracing (from drwxr-xr-x in 15.04 to drwx------ in 15.10), the permissions on /sys/kernel/debug are drwx------ on both 15.10 and 15.04 - which means that /sys/kernel/debug shouldn't be readable from a non-root account on either release, so this looks like a code change in check_disk.
Also note that while 15.10 does change the permissions on /sys/kernel/ debug/tracing (from drwxr-xr-x in 15.04 to drwx------ in 15.10), the permissions on /sys/kernel/debug are drwx------ on both 15.10 and 15.04 - which means that /sys/kernel/debug shouldn't be readable from a non-root account on either release, so this looks like a code change in check_disk.