net: devmem: preserve sockc_err

Preserve the error code returned by sock_cmsg_send and return that on
err.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250523230524.1107879-4-almasrymina@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>