net: hns3: Resolved the issue that the debugfs query result is inconsistent.

This patch modifies the implementation of debugfs:
When the user process stops unexpectedly, not all data of the file system
is read. In this case, the save_buf pointer is not released. When the user
process is called next time, save_buf is used to copy the cached data
to the user space. As a result, the queried data is inconsistent. To solve
this problem, determine whether the function is invoked for the first time
based on the value of *ppos. If *ppos is 0, obtain the actual data.

Fixes: 5e69ea7e ("net: hns3: refactor the debugfs process")
Signed-off-by: Hao Lan <lanhao@huawei.com>
Signed-off-by: Guangwei Zhang <zhangwangwei6@huawei.com>
Signed-off-by: Jijie Shao <shaojijie@huawei.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>