cifs: fix double-free of "string" in cifs_parse_mount_options

Dan reported the following regression in commit d387a5c5:

    + fs/cifs/connect.c:1903 cifs_parse_mount_options() error: double free of 'string'

That patch has some of the new option parsing code free "string" without
setting the variable to NULL afterward. Since "string" is automatically
freed in an error condition, fix the code to just rely on that instead
of freeing it explicitly.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>