block layer: remove a unused argument of drive_stat_acct()

The nr_sector argument of drive_stat_acct() is not used anymore since the read and write sectors statistics are now updated in end_that_request_first(). This patch removes the useless argument.

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>