types: Update obsolete callback_head comment

The comment header for callback_head (and thus for rcu_head) states that
the bottom two bits of a pointer to these structures must be zero.  This
is obsolete:  The new requirement is that only the bottom bit need be
zero.  This commit therefore updates this comment.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>