Fixed scheduler activation (bsc#1052770)

[lslezak]

• Fixes https://bugzilla.suse.com/show_bug.cgi?id=1052770
• Do not activate the new scheduler for devices which do not support it. Activating an unsupported scheduler results in a write error.
• 3.3.0

[coveralls]

Coverage increased (+0.2%) to 3.198% when pulling 7110e5c on scheduler_setup into ab55f04 on master.

[aschnell]

Is the user informed about the failure to set the scheduler for some devices?

[lslezak]

No, this is about setting the default scheduler (via elevator= boot parameter). The kernel also does not report failures for devices which do not support this default at boot.

For a device specific setting an error message would be appropriate, for setting the global default it is IMHO OK to set it only where applicable and silently skip the unsupported devices.

The activation is done just to avoid rebooting the machine, it should produce the same result with the same behavior as reboot.

[aschnell]

LGTM