btrfs: rename page offset parameter in submit_extent_page

We're going to remove sector_t and will use 'offset', so this patch
frees the name.
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -2761,7 +2761,7 @@ static int merge_bio(struct extent_io_tree *tree, struct page *page,
 static int submit_extent_page(unsigned int opf, struct extent_io_tree *tree,
 			      struct writeback_control *wbc,
 			      struct page *page, sector_t sector,
-			      size_t size, unsigned long offset,
+			      size_t size, unsigned long pg_offset,
 			      struct block_device *bdev,
 			      struct bio **bio_ret,
 			      bio_end_io_t end_io_func,
@@ -2785,8 +2785,8 @@ static int submit_extent_page(unsigned int opf, struct extent_io_tree *tree,
 
 		if (prev_bio_flags != bio_flags || !contig ||
 		    force_bio_submit ||
-		    merge_bio(tree, page, offset, page_size, bio, bio_flags) ||
-		    bio_add_page(bio, page, page_size, offset) < page_size) {
+		    merge_bio(tree, page, pg_offset, page_size, bio, bio_flags) ||
+		    bio_add_page(bio, page, page_size, pg_offset) < page_size) {
 			ret = submit_one_bio(bio, mirror_num, prev_bio_flags);
 			if (ret < 0) {
 				*bio_ret = NULL;
@@ -2801,7 +2801,7 @@ static int submit_extent_page(unsigned int opf, struct extent_io_tree *tree,
 	}
 
 	bio = btrfs_bio_alloc(bdev, (u64)sector << 9);
-	bio_add_page(bio, page, page_size, offset);
+	bio_add_page(bio, page, page_size, pg_offset);
 	bio->bi_end_io = end_io_func;
 	bio->bi_private = tree;
 	bio->bi_write_hint = page->mapping->host->i_write_hint;