seeker: backward searching template function

This template turns a forward search primitive (e.g. lower_bound, FIEMAP,
TREE_SEARCH_V2) into a backward search primitive.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

test/Makefile

@@ -7,6 +7,7 @@ PROGRAMS = \
 	path \
 	process \
 	progress \
+	seeker \
 	task \
 
 all: test