pidfs: add missing BUILD_BUG_ON() assert on struct pidfd_info

pidfs: add missing BUILD_BUG_ON() assert on struct pidfd_info

[ Upstream commit d8fc51d8fa3b9894713e7eebcf574bee488fa3e1 ]

Validate that the size of struct pidfd_info is correctly updated.

Link: https://patch.msgid.link/20251028-work-coredump-signal-v1-4-ca449b7b7aa0@kernel.org
Fixes: 1d8db6fd698d ("pidfs, coredump: add PIDFD_INFO_COREDUMP")
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>