Fix getcwd (wrong return value)

kernel/src/syscall/fs.rs

@@ -373,7 +373,7 @@ impl Syscall<'_> {
             return Err(SysError::ERANGE);
         }
         unsafe { util::write_cstr(buf.as_mut_ptr(), &proc.cwd) }
-        Ok(buf.as_ptr() as usize)
+        Ok(proc.cwd.len() + 1)
     }
 
     pub fn sys_lstat(&mut self, path: *const u8, stat_ptr: *mut Stat) -> SysResult {