selftests/bpf: Add bpf_proactive_reclaim test

bpf_proactive_reclaim() performs one bounded reclaim pass per call and,
unlike a write to memory.reclaim, does not retry until the goal is
reached.

Charge 32 MiB of page cache to a cgroup, ask for all of it in one call,
and check that the result is positive but well below the request: one
pass is capped at MEMCG_CHARGE_BATCH pages, far short of 32 MiB on both
4K and 64K page kernels.

The test deliberately covers only the kfunc itself. A full example of
the intended asynchronous use, where a BPF program watches one cgroup's
workingset refaults and reclaims another one from bpf_wq callbacks, is
maintained out of tree at [1], released under GPLv2.

Add CONFIG_MEMCG to the config fragment, without which mm/bpf_memcontrol.c
is not built at all.

[1] https://github.com/teawater/memcg-async-reclaim

Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
Reviewed-by: Barry Song <baohua@kernel.org>
Link: https://lore.kernel.org/bpf/20261009062637.176005-1-hui.zhu@linux.dev
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

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