Btrfs: Fix btrfs_drop_extent_cache for skip pinned case

The check for skip pinned case is wrong, it may breaks the
while loop too soon.
Signed-off-by: NYan Zheng <zheng.yan@oracle.com>
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/file.c

@@ -179,18 +179,14 @@ int btrfs_drop_extent_cache(struct inode *inode, u64 start, u64 end,
 		}
 		flags = em->flags;
 		if (skip_pinned && test_bit(EXTENT_FLAG_PINNED, &em->flags)) {
-			if (em->start <= start &&
-			    (!testend || em->start + em->len >= start + len)) {
+			if (testend && em->start + em->len >= start + len) {
 				free_extent_map(em);
 				write_unlock(&em_tree->lock);
 				break;
 			}
-			if (start < em->start) {
-				len = em->start - start;
-			} else {
-				len = start + len - (em->start + em->len);
 			start = em->start + em->len;
-			}
+			if (testend)
+				len = start + len - (em->start + em->len);
 			free_extent_map(em);
 			write_unlock(&em_tree->lock);
 			continue;