Revert "kernel: freezer should treat PF_IO_WORKER like PF_KTHREAD for freezing"

This reverts commit 15b2219f

.

Before IO threads accepted signals, the freezer using take signals to wake
up an IO thread would cause them to loop without any way to clear the
pending signal. That is no longer the case, so stop special casing
PF_IO_WORKER in the freezer.

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