cifs: fix misspelling of "forcedirectio"

...and add a "directio" synonym since that's what the manpage has
always advertised.

Acked-by: Sachin Prabhu <sprabhu@redhat.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>