nfsd: lock_rename() needs both directories to live on the same fs

... checking that after lock_rename() is too late.  Incidentally,
NFSv2 had no nfserr_xdev...

Fixes: aa387d6c

 "nfsd: fix EXDEV checking in rename"
Cc: stable@vger.kernel.org # v3.9+
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>