nfsd: Fix NFSv4.1/4.2 Secinfo_no_name when security flavors empty

Commit 947bd2479ba9 added support for the Secinfo_no_name operation.
When a non-exported file system is being traversed, the list of
security flavors is empty. It turns out that the Linux client
mount attempt fails when the security flavors list in the
Secinfo_no_name reply is empty.

This patch modifies Secinfo/Secinfo_no_name so that it replies
with all four security flavors when the list is empty.
This fixes Linux NFSv4.1/4.2 mounts when the file system at
the NFSv4 root (as specified on a V4: exports(5) line) is
not exported.

MFC after: 2 weeks