Fix WRITE_STALL start_time (#12147)

Fix WRITE_STALL start_time (#12147)

Summary:
`Delayed` is set true in two cases. One is when `delay` is specified. Other one is in the `while` loop - https://github.com/facebook/rocksdb/blob/cd21e4e69d76ec4ec3b080c8cdae016ac2309cc5/db/db_impl/db_impl_write.cc#L1876
However start_time is not initialized in second case, resulting in time_delayed = immutable_db_options_.clock->NowMicros() - 0(start_time);

Pull Request resolved: https://github.com/facebook/rocksdb/pull/12147

Test Plan: Existing CircleCI

Reviewed By: cbi42

Differential Revision: D52173481

Pulled By: akankshamahajan15

fbshipit-source-id: fb9183b24c191d287a1d715346467bee66190f98