signal: Rename SIL_PERF_EVENT SIL_FAULT_PERF_EVENT for consistency

It helps to know which part of the siginfo structure the siginfo_layout
value is talking about.

v1: https://lkml.kernel.org/r/m18s4zs7nu.fsf_-_@fess.ebiederm.org
v2: https://lkml.kernel.org/r/20210505141101.11519-9-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/87zgumw8cc.fsf_-_@disp2133


Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>