2/2 splice: dont readpage

Splice does not need to readpage to bring the page uptodate before writing
to it, because prepare_write will take care of that for us.

Splice is also wrong to SetPageUptodate before the page is actually uptodate.
This results in the old uninitialised memory leak. This gets fixed as a
matter of course when removing the readpage logic.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>