perf tools: Do not manually count string lengths

Use strlen & macros instead of manually counting string lengths as
this is error prone and may lend to bugs.

Signed-off-by: Vincent Legoll <vincent.legoll@gmail.com>
Cc: Linus Torvalds <torvalds@osdl.org>
LKML-Reference: <4727185d0910130118m5387058dndb02ac9b384af9f0@mail.gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>