tracing: Add hist trigger support for multiple values ('vals=' param)

Allow users to specify trace event fields to use in aggregated sums
via a new 'vals=' keyword.  Before this addition, the only aggregated
sum supported was the implied value 'hitcount'.  With this addition,
'hitcount' is also supported as an explicit value field, as is any
numeric trace event field.

This expands the hist trigger syntax from this:

  # echo hist:keys=xxx [ if filter] > event/trigger

to this:

  # echo hist:keys=xxx:vals=yyy [ if filter] > event/trigger

Link: http://lkml.kernel.org/r/2a5d1adb5ba6c65d7bb2148e379f2fed47f29a68.1457029949.git.tom.zanussi@linux.intel.com

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Tested-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>