Fix intermittency in zfsd degrade tests

tests/sys/cddl/zfs/tests/zfsd/zfsd.kshlib
Sync the file under test before corrupting the vdev, not after.

tests/sys/cddl/zfs/tests/zfsd/zfsd_test.sh
Increase timeouts to 600s. On systems with a slow rootpool, the
extra time is necessary.

Sponsored by: Spectra Logic Corp