loader: bcache: Fix debug printf

Use %j to it works on 64 and 32 bits system.

Reviewed by: imp, tsoome
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33430

(cherry picked from commit 8ed8b4203a2f9b1bd337b8af26021ae2240a0ee8)