bpf: verifier: Support eliding map lookup nullness

This commit allows progs to elide a null check on statically known map
lookup keys. In other words, if the verifier can statically prove that
the lookup will be in-bounds, allow the prog to drop the null check.

This is useful for two reasons:

1. Large numbers of nullness checks (especially when they cannot fail)
   unnecessarily pushes prog towards BPF_COMPLEXITY_LIMIT_JMP_SEQ.
2. It forms a tighter contract between programmer and verifier.

For (1), bpftrace is starting to make heavier use of percpu scratch
maps. As a result, for user scripts with large number of unrolled loops,
we are starting to hit jump complexity verification errors.  These
percpu lookups cannot fail anyways, as we only use static key values.
Eliding nullness probably results in less work for verifier as well.

For (2), percpu scratch maps are often used as a larger stack, as the
currrent stack is limited to 512 bytes. In these situations, it is
desirable for the programmer to express: "this lookup should never fail,
and if it does, it means I messed up the code". By omitting the null
check, the programmer can "ask" the verifier to double check the logic.

Tests also have to be updated in sync with these changes, as the
verifier is more efficient with this change. Notable, iters.c tests had
to be changed to use a map type that still requires null checks, as it's
exercising verifier tracking logic w.r.t iterators.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/68f3ea96ff3809a87e502a11a4bd30177fc5823e.1736886479.git.dxu@dxuuu.xyz


Signed-off-by: Alexei Starovoitov <ast@kernel.org>