pstore: Replace calls to kmalloc and memcpy with kmemdup

Replaced calls to kmalloc and memcpy with a single call to kmemdup. This
patch was found using coccicheck.

Signed-off-by: Alexandru Gheorghiu <gheorghiuandru@gmail.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>