Merge branch 'update-kf_rcu_protected'

Merge branch 'update-kf_rcu_protected'

Kumar Kartikeya Dwivedi says:

====================
Update KF_RCU_PROTECTED

Currently, KF_RCU_PROTECTED only applies to iterator APIs and that too
in a convoluted fashion: the presence of this flag on the kfunc is used
to set MEM_RCU in iterator type, and the lack of RCU protection results
in an error only later, once next() or destroy() methods are invoked on
the iterator. While there is no bug, this is certainly a bit unintuitive,
and makes the enforcement of the flag iterator specific.

In the interest of making this flag useful for other upcoming kfuncs,
e.g. scx_bpf_cpu_curr() [0][1], add enforcement for invoking the kfunc
in an RCU critical section in general.

In addition to this, the aforementioned kfunc also needs to return an
RCU protected pointer, which currently has no generic kfunc flag or
annotation. Add such a flag as well while we are at it.

  [0]: https://lore.kernel.org/all/20250903212311.369697-3-christian.loehle@arm.com
  [1]: https://lore.kernel.org/all/20250909195709.92669-1-arighi@nvidia.com

Changelog:
----------
v2 -> v3
v2: https://lore.kernel.org/bpf/20250917032014.4060112-1-memxor@gmail.com

 * Add back lost hunk reworking documentation for KF_RCU_PROTECTED.

v1 -> v2
v1: https://lore.kernel.org/bpf/20250915024731.1494251-1-memxor@gmail.com

 * Drop KF_RET_RCU and fold change into KF_RCU_PROTECTED. (Andrea, Alexei)
 * Update tests for non-struct pointer return values with KF_RCU_PROTECTED.
====================

Link: https://patch.msgid.link/20250917032755.4068726-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>