VMCI: Check userland-provided datagram size

Ensure that the size filled in by userland in the datagram header
matches the size of the buffer passed down in the IOCTL. Note that we
account for the size of the header itself in the check.

Acked-by: Jorgen Hansen <jhansen@vmware.com>
Acked-by: Aditya Sarwade <asarwade@vmware.com>
Signed-off-by: Andy King <acking@vmware.com>
Reported-by: David Ramos <daramos@stanford.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>