exec: load_script: Do not exec truncated interpreter path

[ Upstream commit b5372fe5dc84235dbe04998efdede3c4daa866a9 ]

Commit 8099b047ecc4 ("exec: load_script: don't blindly truncate
shebang string") was trying to protect against a confused exec of a
truncated interpreter path. However, it was overeager and also refused
to truncate arguments as well, which broke userspace, and it was
reverted. This attempts the protection again, but allows arguments to
remain truncated. In an effort to improve readability, helper functions
and comments have been added.

Co-developed-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Samuel Dionne-Riel <samuel@dionne-riel.com>
Cc: Richard Weinberger <richard.weinberger@gmail.com>
Cc: Graham Christensen <graham@grahamc.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

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