bpf: Generalize bpf_sk_storage

Refactor the functionality in bpf_sk_storage.c so that concept of
storage linked to kernel objects can be extended to other objects like
inode, task_struct etc.

Each new local storage will still be a separate map and provide its own
set of helpers. This allows for future object specific extensions and
still share a lot of the underlying implementation.

This includes the changes suggested by Martin in:

  https://lore.kernel.org/bpf/[email protected]/

adding new map operations to support bpf_local_storage maps:

* storages for different kernel objects to optionally have different
  memory charging strategy (map_local_storage_charge,
  map_local_storage_uncharge)
* Functionality to extract the storage pointer from a pointer to the
  owning object (map_owner_storage_ptr)

Co-developed-by: Martin KaFai Lau <[email protected]>

Signed-off-by: Martin KaFai Lau <[email protected]>
Signed-off-by: KP Singh <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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