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>