ramoops: move spin_lock_init after kmalloc error checking

If cxt->pstore.buf allocated failed, no need to initialize
cxt->pstore.buf_lock. So this patch moves spin_lock_init() after the
error checking.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>