sheepfs: fix volume read/write operation

We should return how many bytes actually written/read from vdi. This fixes a
bug that cat sheepfs/volume/vdi never return.

Reported-by: Marcin Mirosław <marcin@mejor.pl>
Reviewed-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: fix possible NULL defererence by stat_request_begin

After queue_peer_request(), request might be released, so move stat_request_begin
ahead of request handling to avoid possible NULL deferencec.

Reported-by: Marcin Mirosław <marcin@mejor.pl>
Reviewed-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>