binfmt_elf: fix documented return value for load_elf_phdrs()

This function has never returned anything but a plain NULL.

Fixes: 6a8d3894 ("binfmt_elf: Hoist ELF program header loading to a function")
Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/2359389.EDbqzprbEW@mobilepool36.emlix.com