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 <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Reported-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

fs/xfs/xfs_iomap.c

@@ -1087,8 +1087,8 @@ xfs_file_iomap_begin(
 		trace_xfs_iomap_found(ip, offset, length, 0, &imap);
 	}
 
-	if ((flags & IOMAP_WRITE) && xfs_ipincount(ip) &&
-	    (ip->i_itemp->ili_fsync_fields & ~XFS_ILOG_TIMESTAMP))
+	if (xfs_ipincount(ip) && (ip->i_itemp->ili_fsync_fields
+				& ~XFS_ILOG_TIMESTAMP))
 		iomap->flags |= IOMAP_F_DIRTY;
 
 	xfs_bmbt_to_iomap(ip, iomap, &imap);