Merge branch 'enable-static-subprog-calls-in-spin-lock-critical-sections'

Kumar Kartikeya Dwivedi says:

====================
Enable static subprog calls in spin lock critical sections

This set allows a BPF program to make a call to a static subprog within
a bpf_spin_lock critical section. This problem has been hit in sched-ext
and ghOSt [0] as well, and is mostly an annoyance which is worked around
by inling the static subprog into the critical section.

In case of sched-ext, there are a lot of other helper/kfunc calls that
need to be allow listed for the support to be complete, but a separate
follow up will deal with that.

Unlike static subprogs, global subprogs cannot be allowed yet as the
verifier will not explore their body when encountering a call
instruction for them. Therefore, we would need an alternative approach
(some sort of function summarization to ensure a lock is never taken
from a global subprog and all its callees).

 [0]: https://lore.kernel.org/bpf/bd173bf2-dea6-3e0e-4176-4a9256a9a056@google.com

Changelog:
----------
v1 -> v2
v1: https://lore.kernel.org/bpf/20240204120206.796412-1-memxor@gmail.com

 * Indicate global function call in verifier error string (Yonghong, David)
 * Add Acks from Yonghong, David
====================

Link: https://lore.kernel.org/r/20240204222349.938118-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>