Increase the stack space in userspace.

Increase the stack space in userspace.

In 1e33ac1e2677c898a0b5ef6207048c692cb51bf4, the maximum stack size for
userspace tools was set to 8k to mimic the available kernel stack size.

Unfortunately, due to differences in how the stack is used in userspace
vs kernel space, spurious stack overflows could occur in userspace
tools due to the limited stack size. This is especially true in ztest
when debugging is enabled.

This patch multiplies the userspace stack size by 4, which fixes the
stack overflow issues. This comes at the price of not being able to
catch stack size issues in userspace, but the previous solution proved
unreliable anyway.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Fixes #934.