fsdax: replace mmap entry in case of CoW

Replace the existing entry to the newly allocated one in case of CoW. 
Also, we mark the entry as PAGECACHE_TAG_TOWRITE so writeback marks this
entry as writeprotected.  This helps us snapshots so new write pagefaults
after snapshots trigger a CoW.

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

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