Reduce splat_taskq_test2_impl() stack frame size

Reduce splat_taskq_test2_impl() stack frame size

Slightly increasing the size of a kmutex_t has caused us to exceed
the stack frame warning size in splat_taskq_test2_impl().  To address
this the tq_args have been moved to the heap.

  cc1: warnings being treated as errors
  spl-0.6.3/module/splat/splat-taskq.c:358:
  error: the frame size of 1040 bytes is larger than 1024 bytes

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Issue #435