selftests/net: add SO_RESERVE_MEM test

Add a kselftest suite covering SO_RESERVE_MEM behavior:
- Reject unsupported socket types (UDP/SOCK_DGRAM, UNIX/SOCK_STREAM,
  SOCK_RAW, MPTCP) with EOPNOTSUPP, for both zero and non-zero values.
- Validate bounds (negative values and values > SZ_1G return EINVAL).
- Verify reserve adjustments (increase, decrease, page rounding, and
  reset to 0) and corresponding sk_meminfo[SK_MEMINFO_FWD_ALLOC] changes
  via getsockopt(SO_MEMINFO).
- Exercise cgroup v2 memory.max enforcement returning ENOMEM when the
  requested reservation exceeds the cgroup limit.
- Verify that a child socket returned by accept() starts with a 0
  reserve even when its parent listener configured a non-zero
  SO_RESERVE_MEM.
- Verify reserve retention across TCP send/receive/ACK cycles.

Each test runs in private mount and network namespaces. cgroup2 is
mounted on a private tmpfs, so nothing is left behind in the host
filesystem. The test process is moved to a dedicated leaf cgroup, and
back to its original cgroup on teardown. The memory controller is
enabled in the root of the hierarchy if needed, and left enabled like
selftests/cgroup do, as disabling it could break other users.

Also enable CONFIG_CGROUPS=y and CONFIG_MEMCG=y in
tools/testing/selftests/net/config.

Signed-off-by: Eric Dumazet <edumazet@kernel.org>
Link: https://patch.msgid.link/20261001192137.309983-1-edumazet@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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