mm: fix cache mode of dax pmd mappings

track_pfn_insert() in vmf_insert_pfn_pmd() is marking dax mappings as
uncacheable rendering them impractical for application usage.  DAX-pte
mappings are cached and the goal of establishing DAX-pmd mappings is to
attain more performance, not dramatically less (3 orders of magnitude).

track_pfn_insert() relies on a previous call to reserve_memtype() to
establish the expected page_cache_mode for the range.  While memremap()
arranges for reserve_memtype() to be called, devm_memremap_pages() does
not.  So, teach track_pfn_insert() and untrack_pfn() how to handle
tracking without a vma, and arrange for devm_memremap_pages() to
establish the write-back-cache reservation in the memtype tree.

Cc: <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Ross Zwisler <[email protected]>
Cc: Nilesh Choudhury <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Reported-by: Toshi Kani <[email protected]>
Reported-by: Kai Zhang <[email protected]>
Acked-by: Andrew Morton <[email protected]>
Signed-off-by: Dan Williams <[email protected]>

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