Revert "9p: init_in_iov_from_pdu can truncate the size"

This reverts commit 16724a17.
It causes https://bugs.launchpad.net/bugs/1877688

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Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <20200521192627.15259-1-sstabellini@kernel.org>
Signed-off-by: Greg Kurz <groug@kaod.org>