ELF: fix kernel.randomize_va_space double read

ELF loader uses "randomize_va_space" twice. It is sysctl and can change
at any moment, so 2 loads could see 2 different values in theory with
unpredictable consequences.

Issue exactly one load for consistent value across one exec.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Link: https://lore.kernel.org/r/3329905c-7eb8-400a-8f0a-d87cff979b5b@p183


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