rseq/selftests: Clarify rseq_prepare_unload() helper requirements

The rseq.h UAPI now documents that the rseq_cs field must be cleared
before reclaiming memory that contains the targeted struct rseq_cs, but
also that the rseq_cs field must be cleared before reclaiming memory of
the code pointed to by the rseq_cs start_ip and post_commit_offset
fields.

While we can expect that use of dlclose(3) will typically unmap
both struct rseq_cs and its associated code at once, nothing would
theoretically prevent a JIT from reclaiming the code without
reclaiming the struct rseq_cs, which would erroneously allow the
kernel to consider new code which is not a rseq critical section
as a rseq critical section following a code reclaim.

Suggested-by: Florian Weimer <[email protected]>
Signed-off-by: Mathieu Desnoyers <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Florian Weimer <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Peter Zijlstra (Intel) <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: "H . Peter Anvin" <[email protected]>
Cc: Paul Turner <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Signed-off-by: Shuah Khan <[email protected]>

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