Currently we fail if someone requests a valid io scheduler, but it's
modular and not currently loaded. That can happen from a driver init
asking for a different scheduler, or online switching through sysfs
as requested by a user.

This patch makes elevator_get() request_module() to attempt to load
the appropriate module, instead of requiring that done manually.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>