kernel: fix usermode_bootstrap stack mmap error handling

The .map_err().?() pattern doesn't work in a function returning (),
because ? requires the enclosing function to return Result. Replace
with plain .expect() (these are bootstrap initialization failures —
if mmap fails here, the kernel cannot continue).

src/syscall/process.rs

@@ -334,8 +334,7 @@ pub unsafe fn usermode_bootstrap(bootstrap: &Bootstrap, token: &mut CleanLockTok
                             flusher,
                         )?)
                     },
-                )
-                .map_err(|e| format!("failed to mmap bootstrap stack: {e:?}"))?;
+                );
         };
         debug!(
             "Bootstrap stack mapped at {:#x}..{:#x} ({} pages)",