shced:sem: replace mutex with spinlock

The overhead of spinlok is less than mutext (mutex need to call
enter_critical section.)
After this patch, `down_write_trylock` and `down_read_trylock` can be
use in interrupt context.

The instruction is protected with mutex only one instruction so using
spinlock is better.

Signed-off-by: TaiJu Wu <tjwu1217@gmail.com>

include/nuttx/rwsem.h

@@ -25,7 +25,8 @@
  * Included Files
  ****************************************************************************/
 
-#include <nuttx/mutex.h>
+#include <nuttx/semaphore.h>
+#include <nuttx/spinlock.h>
 
 /****************************************************************************
  * Public Type Definitions
@@ -33,11 +34,11 @@
 
 typedef struct
 {
-  mutex_t protected; /* Protecting Locks for Read/Write Locked Tables */
-  sem_t   waiting;   /* Reader/writer Waiting queue */
-  int     waiter;    /* Waiter Count */
-  int     writer;    /* Writer Count */
-  int     reader;    /* Reader Count */
+  spinlock_t protected;
+  sem_t   waiting;
+  int     waiter;
+  int     writer;
+  int     reader;
 } rw_semaphore_t;
 
 /****************************************************************************