Merge branch 'fix-verifier-warning'

Paul Chaignon says:

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The BPF verifier checks the maximum number of call stack frames twice,
first in the main CFG traversal (do_check) and then in a subsequent
traversal (check_max_stack_depth).  If the second check fails, it logs a
'verifier bug' warning and errors out, as the number of call stack frames
should have been verified already.

However, the second check may fail without indicating a verifier bug: if
the excessive function calls reside in dead code, the main CFG traversal
may not visit them; the subsequent traversal visits all instructions,
including dead code.

This case raises the question of how invalid dead code should be treated.
The first patch implements the conservative option and rejects such code;
the second adds a test case.
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Signed-off-by: Alexei Starovoitov <ast@kernel.org>