selftests/bpf: Add test for cgroup storage OOB read

selftests/bpf: Add test for cgroup storage OOB read

Add a test case to reproduce the out-of-bounds read issue when copying
from a cgroup storage map to a pcpu map with a value_size not rounded
up to 8 bytes.

The test creates:
1. A CGROUP_STORAGE map with 4-byte value (not 8-byte aligned)
2. A LRU_PERCPU_HASH map with 4-byte value (same size)

When a socket is created in the cgroup, the BPF program triggers
bpf_map_update_elem() which calls copy_map_value_long(). This function
rounds up the copy size to 8 bytes, but the cgroup storage buffer is
only 4 bytes, causing an OOB read (before the fix).

Signed-off-by: Lang Xu <xulang@uniontech.com>
Link: https://lore.kernel.org/r/D63BF0DBFF1EA122+20260402074236.2187154-2-xulang@uniontech.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>