perf: Fix perf_event_validate_size()

[ Upstream commit 382c27f4ed28f803b1f1473ac2d8db0afc795a1b ]

Budimir noted that perf_event_validate_size() only checks the size of
the newly added event, even though the sizes of all existing events
can also change due to not all events having the same read_format.

When we attach the new event, perf_group_attach(), we do re-compute
the size for all events.

Fixes: a723968c ("perf: Fix u16 overflows")
Reported-by: Budimir Markovic <markovicbudimir@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>