block: make part_in_flight() take an array of two ints

block: make part_in_flight() take an array of two ints

Instead of returning the count that matches the partition, pass
in an array of two ints. Index 0 will be filled with the inflight
count for the partition in question, and index 1 will filled
with the root infligh count, if the partition passed in is not the
root.

This is in preparation for being able to calculate both in one
go.

Signed-off-by: Jens Axboe <axboe@kernel.dk>