Check if sysfs ioscheduler entry is "none"

Check if sysfs ioscheduler entry is "none"

Block devices without elevator (bio-based-dm/loop/etc) have "none"
for sysfs ioscheduler tunable, and write to this entry never fail
from the way elv_iosched_store() is implemented.

This commit checks if the entry is "none" type so as not to show
an irrelevant error message which assumes the entry has a list of
available I/O schedulers.

 # cat /sys/block/dm-0/queue/scheduler
 none
 # echo deadline > /sys/block/dm-0/queue/scheduler ; echo $?
 0
 # cat /sys/block/dm-0/queue/scheduler
 none
 # echo aaa > /sys/block/dm-0/queue/scheduler ; echo $?
 0
 # cat /sys/block/dm-0/queue/scheduler
 none

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>