docs: fix "interruptible" misspellings

A common misspelling of "interruptible" is "interruptable". This fixes
them in the tree and adds the two most common variations to spelling.txt.

Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
17 files changed