io_uring: cleanup io_import_iovec() of pre-mapped request

io_rw_prep_async() goes through a dance of clearing req->io, calling
the iovec import, then re-setting req->io. Provide an internal helper
that does the right thing without needing state tweaked to get there.

This enables further cleanups in io_read, io_write, and
io_resubmit_prep(), but that's left for another time.

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