cifs: potential memory leaks when parsing mnt opts

For example, when mount opt is redundently specified
(e.g., "user=A,user=B,user=C"), kernel kept allocating new key/val
with kstrdup() and overwrite previous ptr (to be freed).

Althouhg mount.cifs in userspace performs a bit of sanitization
(e.g., forcing one user option), current implementation is not
robust. Other options such as iocharset and domainanme are similarly
vulnerable.

Signed-off-by: Taesoo Kim <tsgatesv@gmail.com>
Signed-off-by: Steve French <smfrench@gmail.com>