pidfd: add a new supported_mask field

pidfd: add a new supported_mask field

Some of the future fields in struct pidfd_info can be optional. If the
kernel has nothing to emit in that field, then it doesn't set the flag
in the reply. This presents a problem: There is currently no way to know
what mask flags the kernel supports since one can't always count on them
being in the reply.

Add a new PIDFD_INFO_SUPPORTED_MASK flag and field that the kernel can
set in the reply. Userspace can use this to determine if the fields it
requires from the kernel are supported. This also gives us a way to
deprecate fields in the future, if that should become necessary.

Link: https://patch.msgid.link/20251028-work-coredump-signal-v1-5-ca449b7b7aa0@kernel.org
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>