kernel: ignore mmap result in bootstrap stack setup

The addr_space_lock.mmap() result must be used or explicitly ignored.
Use let _ = ... to discard it — if mmap fails here, the process
crash will be more informative than a Result warning.

src/syscall/process.rs

@@ -318,7 +318,7 @@ pub unsafe fn usermode_bootstrap(bootstrap: &Bootstrap, token: &mut CleanLockTok
         let _base_page = {
             let mut lock_token = token.token();
             let mut addr_space_lock = addr_space.acquire_write(lock_token.downgrade());
-            addr_space_lock
+            let _ = addr_space_lock
                 .mmap(
                     &addr_space,
                     Some(stack_base),