perf: Fix IS_ERR() vs NULL check in inherit_event()

The find_get_pmu_context() returns an ERR_PTR() on failure, we should use
IS_ERR() to check the return value.

Fixes: bd275681 ("perf: Rewrite core context handling")
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20221114091833.1492575-1-cuigaosheng1@huawei.com