bpf: Transfer RCU lock state between subprog calls

Allow transferring an imbalanced RCU lock state between subprog calls
during verification. This allows patterns where a subprog call returns
with an RCU lock held, or a subprog call releases an RCU lock held by
the caller. Currently, the verifier would end up complaining if the RCU
lock is not released when processing an exit from a subprog, which is
non-ideal if its execution is supposed to be enclosed in an RCU read
section of the caller.

Instead, simply only check whether we are processing exit for frame#0
and do not complain on an active RCU lock otherwise. We only need to
update the check when processing BPF_EXIT insn, as copy_verifier_state
is already set up to do the right thing.

Suggested-by: David Vernet <[email protected]>
Tested-by: Yafang Shao <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Signed-off-by: Kumar Kartikeya Dwivedi <[email protected]>
Acked-by: David Vernet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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