NFSD: Return NFS4ERR_FILE_OPEN only when linking an open file

RFC 8881 Section 18.9.4 paragraphs 1 - 2 tell us that RENAME should
return NFS4ERR_FILE_OPEN only when the target object is a file that
is currently open. If the target is a directory, some other status
must be returned.

The VFS is unlikely to return -EBUSY, but NFSD has to ensure that
errno does not leak to clients as a status code that is not
permitted by spec.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>