gnrc_ipv6_nib: wrap mutex lock/unlock in function

sys/net/gnrc/network_layer/ipv6/nib/_nib-internal.h

@@ -266,6 +266,22 @@ extern _nib_dr_entry_t *_prime_def_router;
  */
 void _nib_init(void);
 
+/**
+ * @brief   Acquire exclusive access to the NIB
+ */
+static inline void _nib_acquire(void)
+{
+    mutex_lock(&_nib_mutex);
+}
+
+/**
+ * @brief   Release exclusive access to the NIB
+ */
+static inline void _nib_release(void)
+{
+    mutex_unlock(&_nib_mutex);
+}
+
 /**
  * @brief   Gets interface identifier from a NIB entry
  *