sem_rw.c: coverity: HIS_metric_violation(HIS_GOTO)

This change replaces goto-based control flow with structured if-else blocks
in the down_read() function to comply with MISRA HIS coding standards while
maintaining identical functional behavior.

Signed-off-by: hujun5 <hujun5@xiaomi.com>

sched/semaphore/sem_rw.c

@@ -123,25 +123,25 @@ void down_read(FAR rw_semaphore_t *rwsem)
   if (rwsem->holder == _SCHED_GETTID())
     {
       rwsem->writer++;
-      goto out;
     }
-
+  else
-  while (rwsem->writer > 0)
     {
-      rwsem->waiter++;
+      while (rwsem->writer > 0)
-      nxmutex_unlock(&rwsem->protected);
+        {
-      nxsem_wait(&rwsem->waiting);
+          rwsem->waiter++;
-      nxmutex_lock(&rwsem->protected);
+          nxmutex_unlock(&rwsem->protected);
-      rwsem->waiter--;
+          nxsem_wait(&rwsem->waiting);
+          nxmutex_lock(&rwsem->protected);
+          rwsem->waiter--;
+        }
+
+      /* In a scenario where there is no write lock, we just need to make the
+       * read base +1.
+       */
+
+      rwsem->reader++;
     }
 
-  /* In a scenario where there is no write lock, we just need to make the
-   * read base +1.
-   */
-
-  rwsem->reader++;
-
-out:
   nxmutex_unlock(&rwsem->protected);
 }