spelling.txt: add "unnecessary" typo variants

Add unnecessary typos by copying the necessary typos.

Link: http://lkml.kernel.org/r/1505074722.22023.6.camel@perches.com


Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>