fsdax: introduce dax_lock_mapping_entry()

The current dax_lock_page() locks dax entry by obtaining mapping and index
in page.  To support 1-to-N RMAP in NVDIMM, we need a new function to lock
a specific dax entry corresponding to this file's mapping,index.  And
output the page corresponding to the specific dax entry for caller use.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Shiyang Ruan <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Darrick J. Wong <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Goldwyn Rodrigues <[email protected]>
Cc: Goldwyn Rodrigues <[email protected]>
Cc: Jane Chu <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Ritesh Harjani <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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