selftests/bpf: Exercise kptr map update lifetime

selftests/bpf: Exercise kptr map update lifetime

Add focused map_kptr coverage for BPF-side map updates that touch values
containing referenced kptrs.

The new syscall programs stash the testmod refcounted object in an array
map, a preallocated hash map, and a no-prealloc hash map, then update the
same map from BPF. The refcount must remain elevated after the update,
while the userspace runner destroys the skeleton and reuses the existing
refcount wait to confirm map teardown releases the kptr.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260609202548.3571690-5-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>