ANDROID: cfi: ensure RCU is watching in __cfi_slowpath

Indirect calls can happen when RCU is not watching, so we need to wake
it up again for the CFI shadow and __module_address. As these calls can
happen anywhere, use rcu_nmi_enter() similarly to kernel_text_address(),
and switch to rcu_read_lock_sched() for shadow access.

Bug: 169017431
Change-Id: Iebb857df898e644b4952a62d86fa5ff9852b5711
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>