tools/power/x86/intel-speed-select: Harden daemon pidfile open

tools/power/x86/intel-speed-select: Harden daemon pidfile open

Avoid symlink-based pidfile clobbering by opening the pidfile with
O_NOFOLLOW and validating it with fstat() before locking/writing.

The daemon currently uses a fixed pidfile path under /tmp. A local
unprivileged user can pre-create a symlink at that path and cause a
root-run daemon instance to write into an attacker-chosen file.

Fixes: 7fd786dfbd2c ("tools/power/x86/intel-speed-select: OOB daemon mode")
Signed-off-by: Ali Ahmet MEMIS <dev@unknownbbqr.xyz>
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: stable@kernel.org