io_uring: make the !CONFIG_NET helpers a bit more robust

io_uring: make the !CONFIG_NET helpers a bit more robust

With the prep and prep async split, we now have potentially 3 helpers
that need to be defined for !CONFIG_NET. Add some helpers to do just
that.

Fixes the following compile error on !CONFIG_NET:

fs/io_uring.c:6171:10: error: implicit declaration of function
'io_sendmsg_prep_async'; did you mean 'io_req_prep_async'?
[-Werror=implicit-function-declaration]
   return io_sendmsg_prep_async(req);
             ^~~~~~~~~~~~~~~~~~~~~
	     io_req_prep_async

Fixes: 93642ef88434 ("io_uring: split sqe-prep and async setup")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>