scripts/spelling.txt: add "overwritting" pattern and fix typo instances

Fix typos and add the following to the scripts/spelling.txt:

  overwritting||overwriting

Link: http://lkml.kernel.org/r/1481573103-11329-29-git-send-email-yamada.masahiro@socionext.com
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/drivers/usb/usbip/usbip_common.c b/drivers/usb/usbip/usbip_common.c
index 8b23229..1a6f78d 100644
--- a/drivers/usb/usbip/usbip_common.c
+++ b/drivers/usb/usbip/usbip_common.c
@@ -707,7 +707,7 @@ void usbip_pad_iso(struct usbip_device *ud, struct urb *urb)
 		return;
 
 	/*
-	 * loop over all packets from last to first (to prevent overwritting
+	 * loop over all packets from last to first (to prevent overwriting
 	 * memory when padding) and move them into the proper place
 	 */
 	for (i = np-1; i > 0; i--) {