mm: remove unnecessary WARN_ONCE in page_vma_mapped_walk().

A non present pmd entry can appear after pmd_lock is taken in
page_vma_mapped_walk(), even if THP migration is not enabled.  The
WARN_ONCE is unnecessary.

Link: http://lkml.kernel.org/r/20171003142606.12324-1-zi.yan@sent.com
Fixes: 616b8371 ("mm: thp: enable thp migration in generic path")
Signed-off-by: Zi Yan <zi.yan@cs.rutgers.edu>
Reported-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Tested-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_vma_mapped.c

@@ -167,8 +167,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 						return not_found(pvmw);
 					return true;
 				}
-			} else
-				WARN_ONCE(1, "Non present huge pmd without pmd migration enabled!");
+			}
 			return not_found(pvmw);
 		} else {
 			/* THP pmd was split under us: handle on pte level */