splice: fix problem with current mainline kernels

splice: fix problem with current mainline kernels

splice was updated to work well with the newer vmsplice patches,
where we don't have to copy data at all. But that broke the
regular vmsplice that copies data, which is all the mainline kernel
still support.

Fix that up, so we do appropriate fallbacks. Fixes the problem
reported by gurudas pai <gurudas.pai@oracle.com>

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>