The current histogram expression evaluation logic evaluates the
expression from right to left. This can lead to incorrect results
if the operations are not associative (as is the case for subtraction
and, the now added, division operators).
	e.g. 16-8-4-2 should be 2 not 10 --> 16-8-4-2 = ((16-8)-4)-2
	     64/8/4/2 should be 1 not 16 --> 64/8/4/2 = ((64/8)/4)/2

Division and multiplication are currently limited to single operation
expression due to operator precedence support not yet implemented.

Rework the expression parsing to support the correct evaluation of
expressions containing operators of different precedences; and fix
the associativity error by evaluating expressions with operators of
the same precedence from left to right.

Examples:
        (1) echo 'hist:keys=common_pid:a=8,b=4,c=2,d=1,w=$a-$b-$c-$d' \
                  >> event/trigger
        (2) echo 'hist:keys=common_pid:x=$a/$b/3/2' >> event/trigger
        (3) echo 'hist:keys=common_pid:y=$a+10/$c*1024' >> event/trigger
        (4) echo 'hist:keys=common_pid:z=$a/$b+$c*$d' >> event/trigger

Link: https://lkml.kernel.org/r/20211025200852.3002369-4-kaleshsingh@google.comSigned-off-by: Kalesh Singh <kaleshsingh@google.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>