BACKPORT: perf_event_open: switch to copy_struct_from_user()

(cherry picked from commit c2ba8f41ad366dc12840dd87d8f1565185845126)

Switch perf_event_open() syscall from it's own copying
struct perf_event_attr from userspace to the new dedicated
copy_struct_from_user() helper.

The change is very straightforward, and helps unify the syscall
interface for struct-from-userspace syscalls.

Bug: 261681571
Change-Id: Ib924bab8f4495e81b001136d3cf5db55875c85df
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
[christian.brauner@ubuntu.com: improve commit message]
Link: https://lore.kernel.org/r/20191001011055.19283-5-cyphar@cyphar.com


Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: JohnnLee <johnnlee@google.com>