rust: pin-init: make `[pin_]init_array_from_fn` unwind safe

The previous code only ran cleanup on the explicit error path. If the per-
element initializer panicked partway through, the elements already written
into the array would be leaked: their `Drop` impls would never run. This
violates the pinning requirement.

Fix the unwind safety issue by adding a guard type that drops element on
both error and panic path.

To avoid having to duplicate code between `pin_init_array_from_fn` and the
non-pin variant, extract the code to a shared `ArrayInit` type; this type
is internal and not visible via API.

Reported-by: Gary Guo <gary@garyguo.net>
Closes: https://github.com/Rust-for-Linux/pin-init/issues/136
Signed-off-by: Mirko Adzic <adzicmirko97@gmail.com>
Link: https://patch.msgid.link/20260710-pin-init-sync-v1-6-8fa16cde87ae@garyguo.net
[ Split guard type and the initializer type, move the guard type to be
  within __pinned_init. - Gary ]
Co-developed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Gary Guo <gary@garyguo.net>

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