vhost-user-fs: return correct result of fs_slave_io()

Virtio-fs daemon expects fs_slave_io() returns the number of bytes
read/written on success, but we always return 0 and make userspace think
nothing has been read/written.

Fix it by returning the actual bytes read/written. Note that This
depends on the corresponding fix in vhost crate.

Fixes: #949
Signed-off-by: Eryu Guan <eguan@linux.alibaba.com>

vhost_user_fs/src/fs_cache_req_handler.rs

@@ -42,7 +42,8 @@ impl FsCacheReqHandler for SlaveFsCacheReq {
             VhostUserFSSlaveMsgFlags::MAP_R
         };
 
-        self.fs_slave_map(&msg, fd)
+        self.fs_slave_map(&msg, fd)?;
+        Ok(())
     }
 
     fn unmap(&mut self, requests: Vec<fuse::RemovemappingOne>) -> io::Result<()> {