io_uring/rw: split io_read() into a helper

In the Linux kernel, the following vulnerability has been resolved:

IORING_OP_READ did not correctly consume the provided buffer list when

read i/o returned < 0 (except for -EAGAIN and -EIOCBQUEUED return).

This can lead to a potential use-after-free when the completion via

io_rw_done runs at separate context.