MFC r361699, r361711:

MFC r361699, r361711:

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)
Sponsored by:   iXsystems, Inc.
Differential Revision:  https://reviews.freebsd.org/D25045