vhost/crypto: fix data length check

vhost/crypto: fix data length check

This patch fixes the incorrect data length check to vhost crypto.
Instead of blindly accepting the descriptor length as data length, the
change compare the request provided data length and descriptor length
first. The security issue CVE-2020-14374 is not fixed alone by this
patch, part of the fix is done through:
"vhost/crypto: fix missed request check for copy mode".

CVE-2020-14374
Fixes: 3c79609fda7c ("vhost/crypto: handle virtually non-contiguous buffers")
Cc: stable@dpdk.org
Signed-off-by: Fan Zhang <roy.fan.zhang@intel.com>
Acked-by: Chenbo Xia <chenbo.xia@intel.com>