pidfs: improve ioctl handling

Pidfs supports extensible and non-extensible ioctls. The extensible
ioctls need to check for the ioctl number itself not just the ioctl
command otherwise both backward- and forward compatibility are broken.

The pidfs ioctl handler also needs to look at the type of the ioctl
command to guard against cases where "[...] a daemon receives some
random file descriptor from a (potentially less privileged) client and
expects the FD to be of some specific type, it might call ioctl() on
this FD with some type-specific command and expect the call to fail if
the FD is of the wrong type; but due to the missing type check, the
kernel instead performs some action that userspace didn't expect."
(cf. [1]]

Link: https://lore.kernel.org/r/20250204-work-pidfs-ioctl-v1-1-04987d239575@kernel.org
Link: https://lore.kernel.org/r/CAG48ez2K9A5GwtgqO31u9ZL292we8ZwAA=TJwwEv7wRuJ3j4Lw@mail.gmail.com

 [1]
Fixes: 8ce35281 ("pidfs: check for valid ioctl commands")
Acked-by: Luca Boccassi <luca.boccassi@gmail.com>
Reported-by: Jann Horn <jannh@google.com>
Cc: stable@vger.kernel.org # v6.13; please backport with 8ce35281 ("pidfs: check for valid ioctl commands")
Signed-off-by: Christian Brauner <brauner@kernel.org>