RHBZ: 1539617

Check that, if it is not a boolean, the value the user tries
to write to /proc/fs/cifs/cifsFYI is valid and return an error
if not.
Signed-off-by: NRonnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: NSteve French <stfrench@microsoft.com>
Reported-by: NXiaoli Feng <xifeng@redhat.com>