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>