staging: lustre: use min/max instead of MIN/MAX, simple cases

Custom MIN/MAX operations are being used which are not as robust
as the built in min/max operations which will warn about potentially
problematic type comparisons.

For the simple cases, where no type warning is produced, simply
replace MIN/MAX with min/max.

Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>