rust: allow `clippy::collapsible_if` globally

Similar to `clippy::collapsible_match` (globally allowed in the previous
commit), the `clippy::collapsible_if` lint [1] can make code harder to
read in certain cases.

Thus just let developers decide on their own.

In addition, remove the existing `expect` we had.

Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
Suggested-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/rust-for-linux/DGROP5CHU1QZ.1OKJRAUZXE9WC@garyguo.net/
Link: https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_if [1]
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260426144201.227108-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Makefile

@@ -486,6 +486,7 @@ export rust_common_flags := --edition=2021 \
 			    -Wclippy::as_ptr_cast_mut \
 			    -Wclippy::as_underscore \
 			    -Wclippy::cast_lossless \
+			    -Aclippy::collapsible_if \
 			    -Aclippy::collapsible_match \
 			    -Wclippy::ignored_unit_patterns \
 			    -Aclippy::incompatible_msrv \

drivers/android/binder/range_alloc/array.rs

@@ -204,7 +204,6 @@ impl<T> ArrayRangeAllocator<T> {
         // caller will mark them as unused, which means that they can be freed if the system comes
         // under memory pressure.
         let mut freed_range = FreedRange::interior_pages(offset, size);
-        #[expect(clippy::collapsible_if)] // reads better like this
         if offset % PAGE_SIZE != 0 {
             if i == 0 || self.ranges[i - 1].endpoint() <= (offset & PAGE_MASK) {
                 freed_range.start_page_idx -= 1;