- Rework some of the helper comparison functions for consistency

- Currently trying to refactor all the logic that's different for
extents in the btree iterator code. The main difference is that for non
extents we search for a key greater than or equal to the search key,
while for extents we search for a key strictly greater than the search
key (iter->pos).

So that logic is now handled by btree_iter_search_key(), which computes
the real search key based on iter->pos and whether or not we're
searching for a key >= or > iter->pos.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>