binfmt_elf: Replace offsetof() with struct_size() in fill_note_info()

When dealing with structures containing flexible arrays, struct_size()
provides additional compile-time checks compared to offsetof(). This
enhances code robustness and reduces the risk of potential errors.

Signed-off-by: Xichao Zhao <zhao.xichao@vivo.com>
Link: https://lore.kernel.org/r/20250813115058.635742-1-zhao.xichao@vivo.com


Signed-off-by: Kees Cook <kees@kernel.org>