cfi: Fix __cfi_slowpath_diag RCU usage with cpuidle

cfi: Fix __cfi_slowpath_diag RCU usage with cpuidle

RCU_NONIDLE usage during __cfi_slowpath_diag can result in an invalid
RCU state in the cpuidle code path:

  WARNING: CPU: 1 PID: 0 at kernel/rcu/tree.c:613 rcu_eqs_enter+0xe4/0x138
  ...
  Call trace:
    rcu_eqs_enter+0xe4/0x138
    rcu_idle_enter+0xa8/0x100
    cpuidle_enter_state+0x154/0x3a8
    cpuidle_enter+0x3c/0x58
    do_idle.llvm.6590768638138871020+0x1f4/0x2ec
    cpu_startup_entry+0x28/0x2c
    secondary_start_kernel+0x1b8/0x220
    __secondary_switched+0x94/0x98

Instead, call rcu_irq_enter/exit to wake up RCU only when needed and
disable interrupts for the entire CFI shadow/module check when we do.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20220531175910.890307-1-samitolvanen@google.com
Fixes: cf68fffb66d6 ("add support for Clang CFI")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>