bpf: Fix sk_local_storage diag dumping uninitialized special fields

bpf: Fix sk_local_storage diag dumping uninitialized special fields

Call check_and_init_map_value() after the copy_map_value() to zero out
special field regions. diag_get() copies sk_local_storage map values
into a netlink message using copy_map_value{_locked}(), which
intentionally skip special fields. However, the destination buffer from
nla_reserve_64bit() is not zeroed and the skipped regions contain
uninitialized skb data can be sent to userspace.

Fixes: 1ed4d92458a9 ("bpf: INET_DIAG support in bpf_sk_storage")
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20260423222356.155387-1-ameryhung@gmail.com