btrfs-progs: Do not add extra slash if given path end with it

When use a given path end with a slash like below,
the output of path will have double slash.

Do not add extra slash if there is already one in the given
path.

$ btrfs filesystem du ./test/
output:
Total   Exclusive  Set shared  Filename
0.00B       0.00B           -  /home/gujx/device/tmp/test//foo

Signed-off-by: Gu Jinxiang <gujx@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>

cmds-fi-du.c

@@ -449,7 +449,7 @@ static int du_add_file(const char *filename, int dirfd,
 	}
 
 	pathtmp = pathp;
-	if (pathp == path)
+	if (pathp == path || *(pathp - 1) == '/')
 		ret = sprintf(pathp, "%s", filename);
 	else
 		ret = sprintf(pathp, "/%s", filename);