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

@@ -123,9 +123,9 @@ void down_read(FAR rw_semaphore_t *rwsem)
   if (rwsem->holder == _SCHED_GETTID())
     {
       rwsem->writer++;
-      goto out;
     }
-
+  else
+    {
       while (rwsem->writer > 0)
         {
           rwsem->waiter++;
@@ -140,8 +140,8 @@ void down_read(FAR rw_semaphore_t *rwsem)
        */
 
       rwsem->reader++;
+    }
 
-out:
   nxmutex_unlock(&rwsem->protected);
 }