io_uring: make prep and issue side of req handlers named consistently

Almost all of them are, the odd ones out are the poll remove and the
files update request. Name them like the others, which is:

io_#cmdname_prep	for request preparation
io_#cmdname		for request issue

Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>