staging/lustre/libcfs: use ARRAY_SIZE instead of sizeof/sizeof[0]

use macro definition

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: devel@driverdev.osuosl.org
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>