fsdax: switch the fault handlers to use iomap_iter

Avoid the open coded calls to ->iomap_begin and ->iomap_end and call
iomap_iter instead.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>