perf evsel: Fix write_backwards fallback

Commit b90dc17a5d14 "perf evsel: Add overwrite attribute and check
write_backward" misunderstood the 'order' should be obeyed in
__perf_evsel__open.

But the way this was done for attr.write_backwards was buggy, as we need
to check features in the inverse order of their introduction to the
kernel, so that a newer tool checks first the newest perf_event_attr
fields, detecting that the older kernel doesn't have support for them.

Also, we can avoid calling sys_perf_event_open() if we have already
detected the missing of write_backward.

Cc: He Kuang <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Zefan Li <[email protected]>
Cc: [email protected]
Fixes: b90dc17a5d14 ("perf evsel: Add overwrite attribute and check write_backward")
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Wang Nan <[email protected]>
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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