test/filestore/store_test: add test for 5723

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
HashIndex: reset attr upon split or merge completion

A replay of an in progress merge or split might make
our counts unreliable.

Fixes: #5723
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>