[BLOCK] iosched: fix setting of default io scheduler
[BLOCK] iosched: fix setting of default io scheduler

With the recent reorg of the io scheduler selection, it unfortunately
became possible to select an io scheduler to be the default even if it
wasn't builtin. Fix this by requiring the default scheduler to be
builtin.

Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@suse.de>

KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas