mm/huge_memory.c: deposit a pgtable for DAX PMD faults when required

mm/huge_memory.c: deposit a pgtable for DAX PMD faults when required

Although all architectures use a deposited page table for THP on
anonymous VMAs, some architectures (s390 and powerpc) require the
deposited storage even for file backed VMAs due to quirks of their MMUs.

This patch adds support for depositing a table in DAX PMD fault handling
path for archs that require it.  Other architectures should see no
functional changes.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Oliver O'Halloran <[email protected]>
Cc: Reza Arbab <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: [email protected]
Cc: Oliver O'Halloran <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: Heiko Carstens <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>