Fix data race in scsi cd driver.

There is a data race between cdsysctlinit and cdcheckmedia. Both
functions change softc->flags without synchronization.

Submitted by: Arseny Smalyuk <smalukav@gmail.com>
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D31726

(cherry picked from commit e76786909ce0195855196305a04d86b40f138894)