pidfd: prevent creation of pidfds for kthreads

It's currently possible to create pidfds for kthreads but it is unclear
what that is supposed to mean. Until we have use-cases for it and we
figured out what behavior we want block the creation of pidfds for
kthreads.

Link: https://lore.kernel.org/r/20240731-gleis-mehreinnahmen-6bbadd128383@brauner


Fixes: 32fcb426 ("pid: add pidfd_open()")
Cc: stable@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>