add __load_ioengine() to separate ioengine loading from td context

add __load_ioengine() to separate ioengine loading from td context

Add a sub function __load_ioengine(), which only takes name argument,
to be called from load_ioengine(). No functional changes.

This lets fio_show_ioengine_help() get rid of a local variable td
which only existed to call load_ioengine(&td, ...) while this td had
no actual thread context thus unneeded.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>