Fix rxperf so that it works with pthreads

Fix rxperf so that it works with pthreads

*) Don't allocate 512K on the stack
*) Don't pass stack allocated data into pthread_create()

Reviewed-on: http://gerrit.openafs.org/2854
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit 8c0498699366f7e40253d00604da3d9043b32851)
Change-Id: Iadf2c5512e50aa3f10ee583dddfe5d580e0542bf
Reviewed-on: http://gerrit.openafs.org/2925
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>