uio_hv_generic: increase size of receive and send buffers

When using DPDK there is significant performance boost by using
the largest possible send and receive buffer area.

Unfortunately, with UIO model there is not a good way to configure
this at run time. But it is okay to have a bigger buffer available
even if application only decides to use a smaller piece of it.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>