rfkill: revert back to old userspace API by default

rfkill: revert back to old userspace API by default

Recompiling with the new extended version of struct rfkill_event
broke systemd in *two* ways:
 - It used "sizeof(struct rfkill_event)" to read the event, but
   then complained if it actually got something != 8, this broke
   it on new kernels (that include the updated API);
 - It used sizeof(struct rfkill_event) to write a command, but
   didn't implement the intended expansion protocol where the
   kernel returns only how many bytes it accepted, and errored
   out due to the unexpected smaller size on kernels that didn't
   include the updated API.

Even though systemd has now been fixed, that fix may not be always
deployed, and other applications could potentially have similar
issues.

As such, in the interest of avoiding regressions, revert the
default API "struct rfkill_event" back to the original size.

Instead, add a new "struct rfkill_event_ext" that extends it by
the new field, and even more clearly document that applications
should be prepared for extensions in two ways:
 * write might only accept fewer bytes on older kernels, and
   will return how many to let userspace know which data may
   have been ignored;
 * read might return anything between 8 (the original size) and
   whatever size the application sized its buffer at, indicating
   how much event data was supported by the kernel.

Perhaps that will help avoid such issues in the future and we
won't have to come up with another version of the struct if we
ever need to extend it again.

Applications that want to take advantage of the new field will
have to be modified to use struct rfkill_event_ext instead now,
which comes with the danger of them having already been updated
to use it from 'struct rfkill_event', but I found no evidence
of that, and it's still relatively new.

Cc: stable@vger.kernel.org # 5.11
Reported-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM/Clang v12.0.0-r4 (x86-64)
Link: https://lore.kernel.org/r/20210319232510.f1a139cfdd9c.Ic5c7c9d1d28972059e132ea653a21a427c326678@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>