perf core: Return error pointer if inherit_event() fails to find pmu_ctx

inherit_event() returns NULL only when it finds orphaned events
otherwise it returns either valid child_event pointer or an error
pointer. Follow the same when it fails to find pmu_ctx.

Fixes: bd275681 ("perf: Rewrite core context handling")
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20221118051539.820-1-ravi.bangoria@amd.com