HashIndex: sync top directory during start_split,merge,col_split

Otherwise, the links might be ordered after the in progress
operation tag write.  We need the in progress operation tag to
correctly recover from an interrupted merge, split, or col_split.

Fixes: #5180
Backport: cuttlefish, bobtail
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>