selftests/bpf: Test a multi-slot argument before a struct_ops arena argument

The trampoline reads the __arena flag from the btf_func_model per
argument but stores the ctx one register slot at a time, so the two only
line up if every preceding argument occupies exactly one slot. Every
arena-bearing member of bpf_testmod_ops3 takes single-slot arguments, so
nothing exercises the mapping and a mis-indexed arg_flags lookup would
go unnoticed on any architecture.

Add test_arena_multislot(), whose first argument is a 16-byte struct
passed by value. It fills ctx[0] and ctx[1], putting the arena pointer
at argument index one but slot two. The callback checks both halves of
the struct before dereferencing ctx[2], so a JIT that walks registers
instead of arguments converts the wrong slot and fails the test.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Acked-by: Xu Kuohai <xukuohai@huawei.com>
Link: https://lore.kernel.org/bpf/20260813190356.335181-8-puranjay@kernel.org
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

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