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/[email protected]
Fixes: 616b837153 ("mm: thp: enable thp migration in generic path")
Signed-off-by: Zi Yan <[email protected]>
Reported-by: Abdul Haleem <[email protected]>
Tested-by: Abdul Haleem <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/page_vma_mapped.c

@@ -167,8 +167,7 @@ restart:
 						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 */