splice: change exported internal do_splice() helper to take kernel offset

With the set_fs change, we can no longer rely on copy_{to,from}_user()
accepting a kernel pointer, and it was bad form to do so anyway. Clean
this up and change the internal helper that io_uring uses to deal with
kernel pointers instead. This puts the offset copy in/out in __do_splice()
instead, which just calls the same helper.

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