perf: Add a few assertions

While auditing 6b959ba2 ("perf/core: Fix reentry problem in
perf_output_read_group()") a few spots were found that wanted
assertions.

Notable for_each_sibling_event() relies on exclusion from
modification. This would normally be holding either ctx->lock or
ctx->mutex, however due to how things are constructed disabling IRQs
is a valid and sufficient substitute for ctx->lock.

Another possible site to add assertions would be the various
pmu::{add,del,read,..}() methods, but that's not trivially expressable
in C -- the best option is wrappers, but those are easy enough to
forget.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>