vhost: fix unnecessary dirty page logging

vhost: fix unnecessary dirty page logging

The dirty page logging is only required in vhost enqueue direction for
live migration. This patch removes the unnecessary dirty page logging
in vhost dequeue direction. Otherwise, it will result in a performance
drop. Some if-else judgements are also optimized to improve performance.

Fixes: 6d823bb302c7 ("vhost: prepare sync for descriptor to mbuf refactoring")
Fixes: b6eee3e83402 ("vhost: fix sync dequeue offload")

Signed-off-by: Xuan Ding <xuan.ding@intel.com>
Reviewed-by: Jiayu Hu <jiayu.hu@intel.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Tested-by: Xingguang He <xingguang.he@intel.com>