block: remove precise_iostat

hulk inclusion
category: bugfix
bugzilla: 189480, https://gitee.com/openeuler/kernel/issues/I8Y90H



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Previously, we believed it would be too costly to sum all the percpu
variables 'in_flight' every jiffy if we want to count io accurately. So
we added switch 'precise_iostat' and set it to false be default.

However, the overhead of precise_iostat is
  1. inc and dev in_flight when submitting io and done io
  2. read in_flight up to HZ times per second

It is much lower than our previous evaluation and can be ignored. Now,
turn on the switch by default. And it will be removed in the feature.

Signed-off-by: Li Nan <linan122@huawei.com>