selftests/bpf: add test for softlock when modifying hashmap while iterating

Add test that modifies the map while it's being iterated in such a way that
hangs the kernel thread unless the _safe fix is applied to
bpf_for_each_hash_elem.

Signed-off-by: Brandon Kammerdiener <brandon.kammerdiener@intel.com>
Link: https://lore.kernel.org/r/20250424153246.141677-3-brandon.kammerdiener@intel.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Hou Tao <houtao1@huawei.com>