device-dax: compound devmap support

Use the newly added compound devmap facility which maps the assigned dax
ranges as compound pages at a page size of @align.

dax devices are created with a fixed @align (huge page size) which is
enforced through as well at mmap() of the device.  Faults, consequently
happen too at the specified @align specified at the creation, and those
don't change throughout dax device lifetime.  MCEs unmap a whole dax
huge page, as well as splits occurring at the configured page size.

Performance measured by gup_test improves considerably for
unpin_user_pages() and altmap with NVDIMMs:

  $ gup_test -f /dev/dax1.0 -m 16384 -r 10 -S -a -n 512 -w
  (pin_user_pages_fast 2M pages) put:~71 ms -> put:~22 ms
  [altmap]
  (pin_user_pages_fast 2M pages) get:~524ms put:~525 ms -> get: ~127ms put:~71ms

   $ gup_test -f /dev/dax1.0 -m 129022 -r 10 -S -a -n 512 -w
  (pin_user_pages_fast 2M pages) put:~513 ms -> put:~188 ms
  [altmap with -m 127004]
  (pin_user_pages_fast 2M pages) get:~4.1 secs put:~4.12 secs -> get:~1sec put:~563ms

.. as well as unpin_user_page_range_dirty_lock() being just as effective
as THP/hugetlb[0] pages.

[0] https://lore.kernel.org/linux-mm/20210212130843.13865-5-joao.m.martins@oracle.com/

Link: https://lkml.kernel.org/r/20211202204422.26777-12-joao.m.martins@oracle.com
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Jane Chu <jane.chu@oracle.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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