tools/perf: Fix parallel-perf python script to replace new python syntax ":=" usage

perf test "perf script tests" fails as below in systems
with python 3.6

File "/home/athira/linux/tools/perf/tests/shell/../../scripts/python/parallel-perf.py", line 442
if line := p.stdout.readline():
^
SyntaxError: invalid syntax
--- Cleaning up ---
---- end(-1) ----
92: perf script tests: FAILED!

This happens because ":=" is a new syntax that assigns values
to variables as part of a larger expression. This is introduced
from python 3.8 and hence fails in setup with python 3.6
Address this by splitting the large expression and check the
value in two steps:
Previous line: if line := p.stdout.readline():
Current change:
line = p.stdout.readline()
if line:

With patch

./perf test "perf script tests"
93: perf script tests: Ok

Signed-off-by: Athira Rajeev <[e-mail]>
Acked-by: Adrian Hunter <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Namhyung Kim <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

tools/perf/scripts/python/parallel-perf.py

···
 	pat = re.compile(r"\s*\[[0-9]+\]")
 	p = subprocess.Popen(cmd, stdout=subprocess.PIPE)
 	while True:
-		if line := p.stdout.readline():
+		line = p.stdout.readline()
+		if line:
 			line = line.decode("utf-8")
 			if pat.match(line):
 				line = line.split()