cifs: update mnt_cifs_flags during reconfigure

Many mount flags (e.g. for noperm, noxattr, nobrl,
cifsacl, mfsymlinks and more) can be updated now.

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