vhost: do not inline unlikely fragmented buffers code

vhost: do not inline unlikely fragmented buffers code

Handling of fragmented virtio-net header and indirect descriptors
tables was implemented to fix CVE-2018-1059. It should never
happen with healthy guests and so is already considered as
unlikely code path.

This patch moves these bits into non-inline dedicated functions
to reduce the I-cache pressure.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>