thp: fix comments of __pmd_trans_huge_lock()

To make the comments consistent with the already changed code.

Link: http://lkml.kernel.org/r/1466200004-6196-1-git-send-email-ying.huang@intel.com
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/huge_memory.c

@@ -1458,10 +1458,10 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 }
 
 /*
- * Returns true if a given pmd maps a thp, false otherwise.
+ * Returns page table lock pointer if a given pmd maps a thp, NULL otherwise.
  *
- * Note that if it returns true, this routine returns without unlocking page
- * table lock. So callers must unlock it.
+ * Note that if it returns page table lock pointer, this routine returns without
+ * unlocking page table lock. So callers must unlock it.
  */
 spinlock_t *__pmd_trans_huge_lock(pmd_t *pmd, struct vm_area_struct *vma)
 {