Merge branch 'allow-struct_ops-to-create-map-id-to'

Amery Hung says:

====================
This patchset allows struct_ops implementors to get map id from kdata in
reg(), unreg() and update() so that they create an id to struct_ops
instance mapping. This in turn allows struct_ops kfuncs to refer to the
calling instance without passing a pointer to the struct_ops. The selftest
provides an end-to-end example.

Some struct_ops users extend themselves with other bpf programs, which
also need to call struct_ops kfuncs. For example, scx_layered uses
syscall bpf programs as a scx_layered specific control plane and uses
tracing programs to get additional information for scheduling [0].
The kfuncs may need to refer to the struct_ops instance and perform
jobs accordingly. To allow calling struct_ops kfuncs referring to
specific instances from different program types and context (e.g.,
struct_ops, tracing, async callbacks), the traditional way is to pass
the struct_ops pointer to kfuncs.

This patchset provides an alternative way, through a combination of
bpf map id and global variable. First, a struct_ops implementor will
use the map id of the struct_ops map as the id of an instance. Then,
it needs to maintain an id to instance mapping: inserting a new mapping
during reg() and removing it during unreg(). The map id can be acquired
by calling bpf_struct_ops_id().

v1 -> v2
    Add bpf_struct_ops_id() instead of using bpf_struct_ops_get()

[0] https://github.com/sched-ext/scx/blob/main/scheds/rust/scx_layered/src/bpf/main.bpf.c
====================

Link: https://patch.msgid.link/20250806162540.681679-1-ameryhung@gmail.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

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