obdfs/flushd.c: send writes in FIFO order (use ->prev from list)

obdfs/flushd.c: send writes in FIFO order (use ->prev from list)
    don't get obdo until we have an actual I/O to do
    don't call _brw with zero pages to write
obdfs/rw.c: set jiffies field in pgrq for new page cache entries
obdfs/*.c: change debugging messages