nfsd4: better error return to indicate SSV non-support

As 4.1 becomes less experimental and SSV still isn't implemented, we
have to admit it's not going to be, and return some sensible error
rather than just saying "our server's broken".  Discussion in the ietf
group hasn't turned up any objections to using NFS4ERR_ENC_ALG_UNSUPP
for that purpose.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>