binfmt: Use struct_size()

Use struct_size() instead of hand-writing it. It is less verbose, more
robust and more informative.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/53150beae5dc04dac513dba391a2e4ae8696a7f3.1685290790.git.christophe.jaillet@wanadoo.fr