mm: fix memory leak in copy_huge_pmd()

mm: fix memory leak in copy_huge_pmd()

We allocate a pgtable but do not attach it to anything if the PMD is in
a DAX VMA, causing it to leak.

We certainly try to not free pgtables associated with the huge zero page
if the zero page is in a DAX VMA, so I think this is the right solution.
This needs to be properly audited.

Signed-off-by: Matthew Wilcox <[email protected]>
Cc: Dan Williams <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>