zbd: avoid Coverity defect report

zbd: avoid Coverity defect report

Coverity reported a defect related to the local variable "in_flight":

    Using an unreliable value of "in_flight" inside the second locked
    section. If the data that "in_flight" depends on was changed by
    another thread, this use might be incorrect.

The variable "in_flight" is thread local and other threads can not
change its value. Then the report should be false-positive. Just to
suppress the report, change reference timing of the valuable.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Link: https://lore.kernel.org/r/20230609235914.1376567-1-shinichiro.kawasaki@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>