MAINTAINERS: ksmbd: add cifs_common directory to ksmbd entry

The codes that shared between cifs and ksmbd will move into the cifs_common
directory. This patch add it to the ksmbd entry in the MAINTAINERS file.

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