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

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

  exsits||exists

Link: https://lkml.kernel.org/r/20230126152205.959277-1-luca.ceresoli@bootlin.com


Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>