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

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

  overwrien||overwritten

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Masahiro Yamada <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
7 files changed