nda(4) fix check for Dataset Management support

In the nda(4) driver, only set DISKFLAG_CANDELETE (a.k.a. can support
BIO_DELETE) if the drive supports Dataset Management. There are reports
that without this check, VMWare Workstation does not work reliably.

Fix is to check the ONCS field in the NVMe Controller Data structure for
support. This check previously existed but did not survive the
big-endian changes.

Reported by: yuripv@yuripv.net
Reviewed by: imp, mav, jimharris
Approved by: imp (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D18493