fs: add an item type parameter to next_min

When we are searching the btrfs metadata trees, we usually want only
one type of item.  If the last item in a search result is not of the
desired type, we can restart the search at the next possible key with
that item type, potentially skipping over some uninteresting items we
would otherwise have to fetch, process, and discard.

Also remove a bug in the previous next_min code that would skip over
items if the offset overflowed and the next objectid in the tree had a
lower item type number than the previous objectid.  This doesn't seem
to be a bug that has ever happened, as it would require a file to roll
over in the offset field.

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

include/crucible/fs.h

@@ -192,6 +192,9 @@ namespace crucible {
 		// Copy objectid/type/offset so we move forward
 		void next_min(const BtrfsIoctlSearchHeader& ref);
 
+		// move forward to next object of a single type
+		void next_min(const BtrfsIoctlSearchHeader& ref, const uint8_t type);
+
 		size_t m_buf_size;
 		set<BtrfsIoctlSearchHeader> m_result;
 	};