tracing: Add hist trigger support for user-defined sorting ('sort=' param)

Allow users to specify keys and/or values to sort on.  With this
addition, keys and values specified using the 'keys=' and 'vals='
keywords can be used to sort the hist trigger output via a new 'sort='
keyword.  If multiple sort keys are specified, the output will be
sorted using the second key as a secondary sort key, etc.  The default
sort order is ascending; if the user wants a different sort order,
'.descending' can be appended to the specific sort key.  Before this
addition, output was always sorted by 'hitcount' in ascending order.

This expands the hist trigger syntax from this:

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

to this:

    # echo hist:keys=xxx:vals=yyy:sort=zzz.descending \
          [ if filter] > event/trigger

Link: http://lkml.kernel.org/r/b30a41db66ba486979c4f987aff5fab500ea53b3.1457029949.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>