perf cap: Tidy up and improve capability testing

Remove dependence on libcap. libcap is only used to query whether a
capability is supported, which is just 1 capget system call.

If the capget system call fails, fall back on root permission
checking. Previously if libcap fails then the permission is assumed
not present which may be pessimistic/wrong.

Add a used_root out argument to perf_cap__capable to say whether the
fall back root check was used. This allows the correct error message,
"root" vs "users with the CAP_PERFMON or CAP_SYS_ADMIN capability", to
be selected.

Tidy uses of perf_cap__capable so that tests aren't repeated if capget
isn't supported.

Signed-off-by: Ian Rogers <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Athira Rajeev <[email protected]>
Cc: Changbin Du <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Leo Yan <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Oliver Upton <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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