Apply default security flavor in vfs_export

There may be some version of mountd out there that does not supply a default
security flavor when none is given for an export.

Set the default security flavor in vfs_export if none is given, and remove the
workaround for oexport compat.

Reported by: npn
Reviewed by: rmacklem
Approved by: mav (mentor)
MFC after: 3 days
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D25300