cifs: fix memory leak when password is supplied multiple times

Unlikely but possible. When password is supplied multiple times, we have
to free the previous allocation.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Ashish Sangwan <a.sangwan@samsung.com>
Signed-off-by: Steve French <smfrench@gmail.com>