[RBTREE] Change rbtree off-tree marking in I/O schedulers.

They were abusing the rb_color field to mark nodes which weren't currently
on the tree. Fix that to use the same method as eventpoll did -- setting
the parent pointer to point back to itself. And use the appropriate
accessor macros for setting and reading the parent.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

diff --cc block/as-iosched.c
Simple merge

diff --cc block/cfq-iosched.c
Simple merge

diff --cc block/deadline-iosched.c
Simple merge