perf sched: Use PTHREAD_STACK_MIN to avoid pthread_attr_setstacksize() fail

on ppc64:
/usr/include/bits/local_lim.h:#define PTHREAD_STACK_MIN	131072

therefore following set of commands:

gives:
perf.2.6.37test: builtin-sched.c:493: create_tasks: Assertion `!(err)' failed.

So make sure we do not set stack size lower than PTHREAD_STACK_MIN.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <20110110160417.GB2685@psychotron.brq.redhat.com>
Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>