Assign default security flavor when converting old export args

[freqlabs]

Summary

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: Rick Macklem
Sponsored by: iXsystems, Inc.

[rmacklem]

Looks fine to me.

[cy]

Needs #include <rpc/auth.h>.

[cy]

#include <rpc/rpc.h> is needed, not rpc.auth.h. This of course opens a new can of worms.