fs, dax: unify IOMAP_F_DIRTY read vs write handling policy in the dax core

While reviewing whether MAP_SYNC should strengthen its current guarantee
of syncing writes from the initiating process to also include
third-party readers observing dirty metadata, Dave pointed out that the
check of IOMAP_WRITE is misplaced.

The policy of what to with IOMAP_F_DIRTY should be separated from the
generic filesystem mechanism of reporting dirty metadata. Move this
policy to the fs-dax core to simplify the per-filesystem iomap handlers,
and further centralize code that implements the MAP_SYNC policy. This
otherwise should not change behavior, it just makes it easier to change
behavior in the future.

Reviewed-by: Jan Kara <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Darrick J. Wong <[email protected]>
Cc: Ross Zwisler <[email protected]>
Reported-by: Dave Chinner <[email protected]>
Signed-off-by: Dan Williams <[email protected]>

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