overflow: Correct check_shl_overflow() comment

A 'false' return means the value was safely set, so the comment should
say 'true' for when it is not considered safe.

Cc: Jason Gunthorpe <jgg@ziepe.ca>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Fixes: 0c668477 ("overflow.h: Add arithmetic shift helper")
Link: https://lore.kernel.org/r/20210401160629.1941787-1-kbusch@kernel.org