ExtentWalker: increase efficiency for typical btrfs extent sizes

Perf was blaming more than 50% of cycles on TREE_SEARCH_V2.  strace
showed 4 TREE_SEARCH_V2 calls for every pread in grow_backward().

Fix by increasing the extent fetch batch size so it is more likely
to include the desired items in the first fetch attempt.

This removes TREE_SEARCH_V2 from the top 10 list of cycle consumers.

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

include/crucible/extentwalker.h

@@ -58,7 +58,7 @@ namespace crucible {
 
 		virtual Vec get_extent_map(off_t pos);
 
-		static const unsigned sc_extent_fetch_max = 64;
+		static const unsigned sc_extent_fetch_max = 1024;
 		static const unsigned sc_extent_fetch_min = 4;
 		static const off_t sc_step_size = 0x1000 * (sc_extent_fetch_max / 2);