crucible: reduce buffer size and CPU overhead for BtrfsIoctlSearchKey

We really do need some large buffers for BtrfsIoctlSearchKey in some
cases, but we don't need to zero them out first.  Don't do that so we
save some CPU.

Reduce the default buffer size to 4K because most BISK users don't get
need much more than 1K.  Set the buffer size explicitly to the product of
the number of items and the desired item size in the places that really
need a lot of items.

include/crucible/fs.h

@@ -156,7 +156,7 @@ namespace crucible {
 	ostream & operator<<(ostream &os, const BtrfsIoctlSearchHeader &hdr);
 
 	struct BtrfsIoctlSearchKey : public btrfs_ioctl_search_key {
-		BtrfsIoctlSearchKey(size_t buf_size = 16 * 1024);
+		BtrfsIoctlSearchKey(size_t buf_size = 4096);
 		virtual bool do_ioctl_nothrow(int fd);
 		virtual void do_ioctl(int fd);