vhost: merge sync and async mbuf to descriptor filling

vhost: merge sync and async mbuf to descriptor filling

This patches merges copy_mbuf_to_desc() used by the sync
path with async_mbuf_to_desc() used by the async path.

Most of these complex functions are identical, so merging
them will make the maintenance easier.

In order not to degrade performance, the patch introduces
a boolean function parameter to specify whether it is called
in async context. This boolean is statically passed to this
always-inlined function, so the compiler will optimize this
out.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Jiayu Hu <jiayu.hu@intel.com>