exec: cleanup comments

Remove the second 'from'.
Replace 'backwords' with 'backwards'.
Replace 'visibile' with 'visible'.

Signed-off-by: Tom Rix <trix@redhat.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220211160940.2516243-1-trix@redhat.com