FileStore: fix fd leak in _check_global_replay_guard

Bug introduced in f3f92fe21061e21c8b259df5ef283a61782a44db.

Fixes: #5766
Backport: cuttlefish
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>