perf cap: Remove used_root parameter and simplify capability checks

Refactor perf_cap__capable() to completely remove the used_root out-parameter
as requested by the maintainer. Relying on an explicit used_root boolean
poisoned sequential capability checks (e.g. failing CAP_SYS_ADMIN checks
poisoning the flag for subsequent CAP_PERFMON evaluations for unprivileged
users) and created redundant complexity across check_ftrace_capable(),
symbol__read_kptr_restrict(), and perf_event_paranoid_check().

Streamline the capability API to perform a pure true/false boolean
evaluation. The function checks the Effective set using SYS_capget; if
the syscall is missing or fails on legacy kernels, it cleanly falls back
to checking EUID == 0. This perfectly preserves modern capability-aware host
sessions, guarantees transparent fallback for older kernels, and correctly
rejects privileged operations for containerized root processes that have
explicitly dropped their capability bounding and permitted sets.

Fixes: e25ebda78e23 ("perf cap: Tidy up and improve capability testing")
Suggested-by: Namhyung Kim <namhyung@kernel.org>
Assisted-by: Antigravity:gemini-3.5-flash
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

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