binfmt_elf: simplify error handling in load_elf_phdrs()

The err variable was the same like retval, but capped to <= 0. This is the
same as retval as elf_read() never returns positive values.

Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/4137126.7Qn9TF0dmF@mobilepool36.emlix.com