Use large stacks when available

While stack size will vary by architecture it has historically defaulted to
8K on x86_64 systems. However, as of Linux 3.15 the default thread stack
size was increased to 16K. These kernels are now the default in most non-
enterprise distributions which means we no longer need to assume 8K stacks.

This patch takes advantage of that fact by appropriately reverting stack
conservation changes which were made to ensure stability. Changes which
may have had a negative impact on performance for certain workloads. This
also has the side effect of bringing the code slightly more in line with
upstream.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Closes #4059