cifs: set MAY_SIGN when sec=krb5

Setting this secFlg allows usage of dfs where some servers require
signing and others don't.

Signed-off-by: Martijn de Gouw <martijn.de.gouw@prodrive.nl>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>