Document SPIN_LOCK_UNLOCKED/RW_LOCK_UNLOCKED deprecation

Apparently it's not cool anymore to use SPIN/RW_LOCK_UNLOCKED.  There's
some mention of this in Documentation/spinlocks.txt, but that only talks
about dynamic initialisation.

A comment in the code mentioning the preferred usage would be good IMHO.

[akpm@linux-foundation.org: add reason for deprecation]
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/spinlock_types.h

@@ -85,6 +85,12 @@ typedef struct {
 				RW_DEP_MAP_INIT(lockname) }
 #endif
 
+/*
+ * SPIN_LOCK_UNLOCKED and RW_LOCK_UNLOCKED defeat lockdep state tracking and
+ * are hence deprecated.
+ * Please use DEFINE_SPINLOCK()/DEFINE_RWLOCK() or
+ * __SPIN_LOCK_UNLOCKED()/__RW_LOCK_UNLOCKED() as appropriate.
+ */
 #define SPIN_LOCK_UNLOCKED	__SPIN_LOCK_UNLOCKED(old_style_spin_init)
 #define RW_LOCK_UNLOCKED	__RW_LOCK_UNLOCKED(old_style_rw_init)