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