cifsd: fix potential read overflow in ksmbd_vfs_stream_read()

If *pos or *pos + count is greater than v_len, It will read beyond
the stream_buf buffer. This patch add the check and cut down count with
size of the buffer.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>