rust: drm: fix non-const read8 in unit test

With CONFIG_CC_OPTIMIZE_FOR_SIZE, the address validity check in non-const
`read8` invocaction is not optimized away, leading to build failure.

Fixes: d055768429 ("rust: drm: gem: shmem: Add vmap functions")
Signed-off-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260716142545.3622278-2-gary@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/drm/gem/shmem.rs

@@ -688,7 +688,7 @@ fn vmap_io() -> Result {
         // Ensure the ordering in memory is correct
         let expected = 0xFEDCBA98_u32.to_ne_bytes().into_iter();
         for (offset, expected) in (0x20..=0x23).zip(expected) {
-            assert_eq!(vmap.read8(offset), expected);
+            assert_eq!(vmap.try_read8(offset).unwrap(), expected);
         }
 
         Ok(())