nfsd: Add support for the NFSv4.1/4.2 Secinfo_no_name operation

The Linux client is now attempting to use the Secinfo_no_name
operation for NFSv4.1/4.2 mounts. Although it does not seem to
mind the NFSERR_NOTSUPP reply, adding support for it seems
reasonable.

I also noticed that "savflag" needed to be 64bits in
nfsrvd_secinfo() since nd_flag in now 64bits, so I changed
the declaration of it there. I also added code to set "vp" NULL
after performing Secinfo/Secinfo_no_name, since these
operations consume the current FH, which is represented
by "vp" in nfsrvd_compound().

Fixing when the server replies NFSERR_WRONGSEC so that
it conforms to RFC5661 Sec. 2.6 still needs to be done
in a future commit.

MFC after: 2 weeks