[grpcvtctldserver] Fix backup detail limit math
[grpcvtctldserver] Fix backup detail limit math

We always want to base off the total number of backups, not the total
number after applying the overall limit.

Signed-off-by: Andrew Mason <8a240ff75aa1467ba5eba4db0c8200b61262547e@slack-corp.com>

mahak/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,tinyspeck/vitess,vitessio/vitess,tinyspeck/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,tinyspeck/vitess,tinyspeck/vitess,mahak/vitess,tinyspeck/vitess,vitessio/vitess,tinyspeck/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,tinyspeck/vitess,mahak/vitess