rust: pin-init: internal: move alignment check to `make_field_check`

rust: pin-init: internal: move alignment check to `make_field_check`

Instead of having the reference creation serving dual-purpose as both for
let bindings and alignment check, detangle them so that the alignment check
is done explicitly in `make_field_check`. This is more robust against
refactors that may change the way let bindings are created.

Cc: stable@vger.kernel.org
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260427-pin-init-fix-v3-1-496a699674dd@garyguo.net
[ Reworded for typo. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>