cifs: return error on invalid value written to cifsFYI

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