rcu/nocb: reduce stack usage in nocb_gp_wait()

When CONFIG_UBSAN_ALIGNMENT is enabled, the stack usage of nocb_gp_wait()
grows above typical warning limits:

In file included from kernel/rcu/tree.c:4930:
kernel/rcu/tree_nocb.h: In function 'rcu_nocb_gp_kthread':
kernel/rcu/tree_nocb.h:866:1: error: the frame size of 1968 bytes is larger than 1280 bytes [-Werror=frame-larger-than=]

Apparently, the problem is passing rcu_data from a 'void *' pointer,
which gcc assumes may be misaligned. When the function is not inlined
into rcu_nocb_gp_kthread(), that is no longer visible to gcc.

Add a 'noinline_for_stack' annotation that leads to skipping a lot of
the alignment sanitizer checks and keeps the stack usage 60% lower here.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>