rcu: Make SRCU mandatory

Kernels configured with CONFIG_PRINTK=n and CONFIG_SRCU=n get build
failures.  This causes trouble for deep embedded systems.  But given
that there are more than 25 instances of "select SRCU" in the kernel,
it is hard to believe that there are many kernels running in production
without SRCU.  This commit therefore makes SRCU mandatory.  The SRCU
Kconfig option remains for backwards compatibility, and will be removed
when it is no longer used.

[ paulmck: Update per kernel test robot feedback. ]

Reported-by: John Ogness <john.ogness@linutronix.de>
Reported-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: <linux-arch@vger.kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Reviewed-by: John Ogness <john.ogness@linutronix.de>