btrfs: Fix the wrong condition judgment about subset extent map

Previous commit: btrfs: Fix and enhance merge_extent_mapping() to insert
best fitted extent map
is using wrong condition to judgement whether the range is a subset of a
existing extent map.

This may cause bug in btrfs no-holes mode.

This patch will correct the judgment and fix the bug.
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/inode.c

@@ -6586,7 +6586,7 @@ struct extent_map *btrfs_get_extent(struct inode *inode, struct page *page,
 		 * extent causing the -EEXIST.
 		 */
 		if (start >= extent_map_end(existing) ||
-		    start + len <= existing->start) {
+		    start <= existing->start) {
 			/*
 			 * The existing extent map is the one nearest to
 			 * the [start, start + len) range which overlaps