perf_counter: add more context information

Change the callchain context entries to u16, so as to gain some space.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
LKML-Reference: <20090406094517.457320003@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>