mm/thp/pagecache/collapse: free the pte page table on collapse for thp page cache.

With THP page cache, when trying to build a huge page from regular pte
pages, we just clear the pmd entry.  We will take another fault and at
that point we will find the huge page in the radix tree, thereby using
the huge page to complete the page fault

The second fault path will allocate the needed pgtable_t page for archs
like ppc64.  So no need to deposit the same in collapse path.
Depositing them in the collapse path resulting in a pgtable_t memory
leak also giving errors like

  BUG: non-zero nr_ptes on freeing mm: 3

Fixes: 953c66c2b22a ("mm: THP page cache support for ppc64")
Link: http://lkml.kernel.org/r/20161212163428.6780-2-aneesh.kumar@linux.vnet.ibm.com
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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