iomap: mark the iomap argument to iomap_read_page_sync const

iomap_read_page_sync never modifies the passed in iomap, so mark
it const.

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>