BKL: introduce CONFIG_BKL.

With all the patches we have queued in the BKL removal tree, only a
few dozen modules are left that actually rely on the BKL, and even
there are lots of low-hanging fruit. We need to decide what to do
about them, this patch illustrates one of the options:

Every user of the BKL is marked as 'depends on BKL' in Kconfig,
and the CONFIG_BKL becomes a user-visible option. If it gets
disabled, no BKL using module can be built any more and the BKL
code itself is compiled out.

The one exception is file locking, which is practically always
enabled and does a 'select BKL' instead. This effectively forces
CONFIG_BKL to be enabled until we have solved the fs/lockd
mess and can apply the patch that removes the BKL from fs/locks.c.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
diff --git a/init/Kconfig b/init/Kconfig
index 2de5b1c..2005a1d 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -64,7 +64,7 @@
 
 config LOCK_KERNEL
 	bool
-	depends on SMP || PREEMPT
+	depends on (SMP || PREEMPT) && BKL
 	default y
 
 config INIT_ENV_ARG_LIMIT