Assign default security flavor when converting old export args

Assign default security flavor when converting old export args

vfs_export requires security flavors be explicitly listed when
exporting as of r360900.

Use the default AUTH_SYS flavor when converting old export args to
ensure compatibility with the legacy mount syscall.

Reported by:	rmacklem
Reviewed by:	rmacklem
Approved by:	mav (mentor)
MFC after:	3 days
Sponsored by:	iXsystems, Inc.
Differential Revision:	https://reviews.freebsd.org/D25045