Nexus: free bios when we're done with them
Nexus: free bios when we're done with them

- Fix failure to decrement bio refcount in chunk store endio function.
This was causing a kernel memory leak of 929 bytes per chunk store I/O
operation, visible in the bio and biovec slabs in /proc/slabinfo.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr