Apply default security flavor in vfs_export

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