NFS: fix nfs_fetch_iversion()

[ Upstream commit b876d708 ]

The change attribute is always set by all NFS client versions so get rid
of the open-coded version.

Fixes: 3cc55f44 ("nfs: use change attribute for NFS re-exports")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>