convert do_preadv()/do_pwritev()

fdput() can be transposed with add_{r,w}char() and inc_sysc{r,w}();
it's the same story as with do_readv()/do_writev(), only with
fdput() instead of fdput_pos().

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>