engines/io_uring: pass back correct error value when interrupted

engines/io_uring: pass back correct error value when interrupted

Running with an io_uring engine and using a USR1 signal to show
current status will end up terminating the job with:

fio: pid=91726, err=-4/file:ioengines.c:320, func=get_events, error=Unknown error -4
sfx: (groupid=0, jobs=1): err=-4 (file:ioengines.c:320, func=get_events, error=Unknown error -4): pid=91726: Wed Aug 24 11:59:51 2022

Ensure that the return value is set correctly based on the errno.

Signed-off-by: Jens Axboe <axboe@kernel.dk>