mm: remove unnecessary WARN_ONCE in page_vma_mapped_walk().

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: 616b8371539a ("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]>