rcu: Make SRCU optional by using CONFIG_SRCU

SRCU is not necessary to be compiled by default in all cases. For tinification
efforts not compiling SRCU unless necessary is desirable.

The current patch tries to make compiling SRCU optional by introducing a new
Kconfig option CONFIG_SRCU which is selected when any of the components making
use of SRCU are selected.

If we do not select CONFIG_SRCU, srcu.o will not be compiled at all.

   text    data     bss     dec     hex filename
   2007       0       0    2007     7d7 kernel/rcu/srcu.o

Size of arch/powerpc/boot/zImage changes from

   text    data     bss     dec     hex filename
 831552   64180   23944  919676   e087c arch/powerpc/boot/zImage : before
 829504   64180   23952  917636   e0084 arch/powerpc/boot/zImage : after

so the savings are about ~2000 bytes.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Josh Triplett <josh@joshtriplett.org>
CC: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: resolve conflict due to removal of arch/ia64/kvm/Kconfig. ]
diff --git a/drivers/clk/Kconfig b/drivers/clk/Kconfig
index 3f44f29..91f8613 100644
--- a/drivers/clk/Kconfig
+++ b/drivers/clk/Kconfig
@@ -13,6 +13,7 @@
 	bool
 	select HAVE_CLK_PREPARE
 	select CLKDEV_LOOKUP
+	select SRCU
 	---help---
 	  The common clock framework is a single definition of struct
 	  clk, useful across many platforms, as well as an
diff --git a/drivers/cpufreq/Kconfig b/drivers/cpufreq/Kconfig
index 29b2ef5..a171fef 100644
--- a/drivers/cpufreq/Kconfig
+++ b/drivers/cpufreq/Kconfig
@@ -2,6 +2,7 @@
 
 config CPU_FREQ
 	bool "CPU Frequency scaling"
+	select SRCU
 	help
 	  CPU Frequency scaling allows you to change the clock speed of 
 	  CPUs on the fly. This is a nice method to save power, because 
diff --git a/drivers/devfreq/Kconfig b/drivers/devfreq/Kconfig
index faf4e70..3891f67 100644
--- a/drivers/devfreq/Kconfig
+++ b/drivers/devfreq/Kconfig
@@ -1,5 +1,6 @@
 menuconfig PM_DEVFREQ
 	bool "Generic Dynamic Voltage and Frequency Scaling (DVFS) support"
+	select SRCU
 	help
 	  A device may have a list of frequencies and voltages available.
 	  devfreq, a generic DVFS framework can be registered for a device
diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 5bdedf6..c355a22 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -5,6 +5,7 @@
 menuconfig MD
 	bool "Multiple devices driver support (RAID and LVM)"
 	depends on BLOCK
+	select SRCU
 	help
 	  Support multiple physical spindles through a single logical device.
 	  Required for RAID and logical volume management.
diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
index d6607ee..84673eb 100644
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -197,6 +197,7 @@
 
 config NETPOLL
 	def_bool NETCONSOLE
+	select SRCU
 
 config NET_POLL_CONTROLLER
 	def_bool NETPOLL