pid: optional first-fit pid allocation

pid: optional first-fit pid allocation

Noone would need to use this allocation strategy (it's slower, pid
numbers collide sooner). Its primary purpose are pid namespaces in
conjunction with pids.max cgroup limit which keeps (virtual) pid numbers
below the given limit. This is for 32-bit userspace programs that may
not work well with pid numbers above 65536.

Link: https://lore.kernel.org/r/20241122132459.135120-1-aleksandr.mikhalitsyn@canonical.com/
Link: https://lkml.kernel.org/r/20250221170249.890014-3-mkoutny@suse.com
Cc: Alexander Mikhalitsyn <alexander@mihalicyn.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kees Cook <kees@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>