fs/fat: Clean up open-coded sector <-> cluster conversions

Use the clust_to_sect() helper that was introduced earlier, and add an
inverse sect_to_clust(), plus update the various spots that open-coded
this conversion previously.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/fs/fat/fat_write.c b/fs/fat/fat_write.c
index d0468ba..9d2e0ed 100644
--- a/fs/fat/fat_write.c
+++ b/fs/fat/fat_write.c
@@ -502,8 +502,7 @@
 	int ret;
 
 	if (clustnum > 0)
-		startsect = mydata->data_begin +
-				clustnum * mydata->clust_size;
+		startsect = clust_to_sect(mydata, clustnum);
 	else
 		startsect = mydata->rootdir_sect;
 
@@ -751,8 +750,7 @@
 	__u32 startsect, sect_num, offset;
 
 	if (clustnum > 0) {
-		startsect = mydata->data_begin +
-				clustnum * mydata->clust_size;
+		startsect = clust_to_sect(mydata, clustnum);
 	} else {
 		startsect = mydata->rootdir_sect;
 	}
@@ -791,7 +789,7 @@
 static dir_entry *find_directory_entry(fsdata *mydata, int startsect,
 	char *filename, dir_entry *retdent, __u32 start)
 {
-	__u32 curclust = (startsect - mydata->data_begin) / mydata->clust_size;
+	__u32 curclust = sect_to_clust(mydata, startsect);
 
 	debug("get_dentfromdir: %s\n", filename);