mm/mmap_lock: Remove unnecessary 'NULL' values from Pointer

Pointer variables allocate memory first, and then judge. There is no
need to initialize the assignment.

Signed-off-by: XU pengfei <xupengfei@nfschina.com>

mm/mmap_lock.c

@@ -198,7 +198,7 @@ void trace_mmap_lock_unreg(void)
  */
 static const char *get_mm_memcg_path(struct mm_struct *mm)
 {
-	char *buf = NULL;
+	char *buf;
 	struct mem_cgroup *memcg = get_mem_cgroup_from_mm(mm);
 
 	if (memcg == NULL)