Btrfs: cleanup the same name in end_bio_extent_readpage

We've defined a 'offset' out of bio_for_each_segment_all.

This is just a clean rename, no function changes.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/extent_io.c

@@ -2543,12 +2543,12 @@ static void end_bio_extent_readpage(struct bio *bio, int err)
 		if (likely(uptodate)) {
 			loff_t i_size = i_size_read(inode);
 			pgoff_t end_index = i_size >> PAGE_CACHE_SHIFT;
-			unsigned offset;
+			unsigned off;
 
 			/* Zero out the end if this page straddles i_size */
-			offset = i_size & (PAGE_CACHE_SIZE-1);
-			if (page->index == end_index && offset)
-				zero_user_segment(page, offset, PAGE_CACHE_SIZE);
+			off = i_size & (PAGE_CACHE_SIZE-1);
+			if (page->index == end_index && off)
+				zero_user_segment(page, off, PAGE_CACHE_SIZE);
 			SetPageUptodate(page);
 		} else {
 			ClearPageUptodate(page);