rbtree: Add const qualifier to some functions.

The 'rb_first()', 'rb_last()', 'rb_next()' and 'rb_prev()' calls take a pointer
to an RB node or RB root. They do not change the pointed objects, so add a
'const' qualifier.

See commit f4b477c47332367d35686bd2b808c2156b96d7c7 in the linux source tree.