seqlock: Remove memory barrier if Non-SMP.

This commit removed unnecessary memory barriers of the seqlock
implementation, since they may block the CPU pipeline and lead to
performance degradation.

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

include/nuttx/seqlock.h

@@ -104,7 +104,10 @@ unsigned int read_seqbegin(FAR const seqcount_t *s)
 
   while ((seq = s->sequence) & 1);
 
+#ifdef CONFIG_SMP
   UP_DMB();
+#endif
+
   return seq;
 }
 
@@ -129,7 +132,10 @@ unsigned int read_seqretry(FAR const seqcount_t *s, unsigned int start)
 {
   unsigned int seq;
 
+#ifdef CONFIG_SMP
   UP_DMB();
+#endif
+
   seq = s->sequence;
   return seq != start;
 }