rust: pin-init: use irrefutable pattern for `stack_pin_init`

In Rust 1.100.0, `Infallible` will become an alias of `!`. The let
binding in `stack_pin_init` will thus become unreachable and produce
an "unreachable expression" warning for the subsequent match, and thus
will fail a `-Dwarnings` build. For this macro, all we need to know is
that the error type is uninhabited, so replace this with an irrefutable
pattern instead.

[ The error looks like (dummy reproducer):

      error: unreachable expression
         --> rust/kernel/sync.rs:177:5
          |
      177 |     pin_init::stack_pin_init!(let num = 42u32);
          |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
          |     |
          |     unreachable expression
          |     any code following this expression is unreachable
          |
          = note: `-D unreachable-code` implied by `-D warnings`
          = help: to override `-D warnings` add `#[allow(unreachable_code)]`
          = note: this error originates in the macro `pin_init::stack_pin_init` (in Nightly builds, run with -Z macro-backtrace for more info)

    - Miguel ]

Reported-by: Mohamad Alsadhan <mo@sdhn.cc>
Closes: https://github.com/Rust-for-Linux/pin-init/pull/171
Signed-off-by: Gary Guo <gary@garyguo.net>
Cc: stable@vger.kernel.org # Needed in 7.1.y and later (for 6.12.y and 6.18.y a custom one is needed).
Link: https://patch.msgid.link/20260828155033.2101924-1-gary@kernel.org
[ Reworded for typos. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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