lib/interval-tree: correct comment wrt generic flavor

interval_tree.h _is_ the generic flavor.

Link: http://lkml.kernel.org/r/20170719014603.19029-13-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>