cifs: show the "w" bit for writeable /proc/fs/cifs/* files

RHBZ: 1539612

Lets show the "w" bit for those files have a .write interface to set/enable/...
the feature.

Reported-by: Xiaoli Feng <xifeng@redhat.com>
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>