Add default stack checking

Add default stack checking

When your kernel is built with kernel stack tracing enabled and you
have the debugfs filesystem mounted.  Then the zfs.sh script will clear
the worst observed kernel stack depth on module load and check the worst
case usage on module removal.  If the stack depth ever exceeds 7000
bytes the full stack will be printed for debugging.  This is dangerously
close to overrunning the default 8k stack.

This additional advisory debugging is particularly valuable when running
the regression tests on a kernel built with 16k stacks.  In this case,
almost no matter how bad the stack overrun is you will see be able to
get a clean stack trace for debugging.  Since the worst case stack usage
can be highly variable it's helpful to always check the worst case usage.