selftests/pidfd: test that fields beyond the struct size are not reported

PIDFD_GET_INFO must not set the mask bit of a field that lies beyond
the struct size encoded in the ioctl command. Add a test that requests
the coredump fields and the supported mask with PIDFD_INFO_SIZE_VER0
over a full-size, poisoned struct, and checks that the mask names none
of those fields and that the poison behind the first 64 bytes is
intact.

Add fallback definitions of PIDFD_INFO_SIZE_VER0 to VER3 to pidfd.h,
like the other uapi constants there.

Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Link: https://patch.msgid.link/20261024041334.831273-3-nickolai@csail.mit.edu
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>