selftests/bpf: avoid mark_all_scalars_precise() trigger in one of iter tests

iter_pass_iter_ptr_to_subprog subtest is relying on actual array size
being passed as subprog parameter. This combined with recent fixes to
precision tracking in conditional jumps ([0]) is now causing verifier to
backtrack all the way to the point where sum() and fill() subprogs are
called, at which point precision backtrack bails out and forces all the
states to have precise SCALAR registers. This in turn causes each
possible value of i within fill() and sum() subprogs to cause
a different non-equivalent state, preventing iterator code to converge.

For now, change the test to assume fixed size of passed in array. Once
BPF verifier supports precision tracking across subprogram calls, these
changes will be reverted as unnecessary.

  [0] 71b547f56124 ("bpf: Fix incorrect verifier pruning due to missing register precision taints")

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230424235128.1941726-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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