iomap: constify iomap_iter_srcmap

The srcmap returned from iomap_iter_srcmap is never modified, so mark
the iomap returned from it const and constify a lot of code that never
modifies the iomap.

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>