Currently, deferred flushes' info is striped between several lists in
the flush tables. Instead, move all information about a specific flush
to a single entry in this table.

This patch does not introduce any functional change.
Signed-off-by: NOmer Peleg <omer@cs.technion.ac.il>
[mad@cs.technion.ac.il: rebased and reworded the commit message]
Signed-off-by: NAdam Morrison <mad@cs.technion.ac.il>
Reviewed-by: NShaohua Li <shli@fb.com>
Reviewed-by: NBen Serebrin <serebrin@google.com>
Signed-off-by: NDavid Woodhouse <David.Woodhouse@intel.com>