perf script: force stdin for flamegraph in live mode

perf script: force stdin for flamegraph in live mode

Currently, running "perf script flamegraph -a -F 99 sleep 1" should
produce flamegraph.html containing the flamegraph. Howevever, it gives a
segmentation fault.

This is caused because the flamegraph.py script is
supposed to take as input the output of "perf record", which should be
in stdin. This would require passing "-i -" to flamegraph.py. However,
the "flamegraph-report" script causes "perf script" command to take the
"-i -" option instead of flamegraph.py, which causes no problem for
"perf script", but causes a seg fault since flamegraph.py has no input
file. To fix this I added the "-i -" option directly to the
flamegraph-report script to ensure flamegraph.py gets input from stdin.

Signed-off-by: Anubhav Shelat <ashelat@redhat.com>
Tested-by: Michael Petlan <mpetlan@redhat.com>
Link: https://lore.kernel.org/r/20250131145704.3164542-2-ashelat@redhat.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>