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>

fs/iomap/buffered-io.c

@@ -531,7 +531,7 @@ iomap_write_failed(struct inode *inode, loff_t pos, unsigned len)
 
 static int
 iomap_read_page_sync(loff_t block_start, struct page *page, unsigned poff,
-		unsigned plen, struct iomap *iomap)
+		unsigned plen, const struct iomap *iomap)
 {
 	struct bio_vec bvec;
 	struct bio bio;