Limit maximum object size in kmem tests

Limit the maximum object size to 1/128 of total system memory for
the kmem cache tests. Large values can result in out of memory errors
for systems with less the 512M of memory. Additionally, use the
known number of objects per-slab for calculating the number of
objects to use for a test.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>