nfsd4: don't query change attribute in v2/v3 case

inode_query_iversion() has side effects, and there's no point calling it
when we're not even going to use it.

We check whether we're currently processing a v4 request by checking
fh_maxsize, which is arguably a little hacky; we could add a flag to
svc_fh instead.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>