nfsd: don't allow setting maxblksize after svc created

It's harmless to set this after the server is created, but also
ineffective, since the value is only used at the time of
svc_create_pooled().  So fail the attempt, in keeping with the pattern
set by write_versions, write_{lease,grace}time and write_recoverydir.

(This could break userspace that tried to write to nfsd/max_block_size
between setting up sockets and starting the server.  However, such code
wouldn't have worked anyway, and I don't know of any examples--rpc.nfsd
in nfs-utils, probably the only user of the interface, doesn't do that.)

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