ksmbd: fix stream write failure

If there is no stream data in file, v_len is zero.
So, If position(*pos) is zero, stream write will fail
due to stream write position validation check.
This patch reorganize stream write position validation.

Fixes: 0ca6df4f ("ksmbd: prevent out-of-bounds stream writes by validating *pos")
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>