Merge branch 'Introduce access remote cpu elem support in BPF percpu map'

Merge branch 'Introduce access remote cpu elem support in BPF percpu map'

Feng zhou says:

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From: Feng Zhou <[email protected]>

Trace some functions, such as enqueue_task_fair, need to access the
corresponding cpu, not the current cpu, and bpf_map_lookup_elem percpu map
cannot do it. So add bpf_map_lookup_percpu_elem to accomplish it for
percpu_array_map, percpu_hash_map, lru_percpu_hash_map.

v1->v2: Addressed comments from Alexei Starovoitov.
- add a selftest for bpf_map_lookup_percpu_elem.
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Signed-off-by: Alexei Starovoitov <[email protected]>