perf/core: Fix unconditional security_locked_down() call

In the Linux kernel, the following vulnerability has been resolved:

perf/core: Fix unconditional security_locked_down() call

Currently, the lockdown state is queried unconditionally, even though

its result is used only if the PERF_SAMPLE_REGS_INTR bit is set in

attr.sample_type. While that doesn't matter in case of the Lockdown LSM,

it causes trouble with the SELinux's lockdown hook implementation.

SELinux implements the locked_down hook with a check whether the current

task's type has the corresponding "lockdown" class permission

("integrity" or "confidentiality") allowed in the policy. This means

that calling the hook when the access control decision would be ignored

generates a bogus permission check and audit record.

Fix this by checking sample_type first and only calling the hook when

its result would be honored.