perf probe: Usability fixes

Ingo pointed out few perf probe usability related errors during his
review of uprobes.

Since these issues are independent of uprobes, fixing them in a separate
patch.

Suggested-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Link: http://lkml.kernel.org/r/20120120121354.GL15447@linux.vnet.ibm.com


Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>