NFSv4.1/flexfiles: Fix a protocol error in layoutreturn

According to the flexfiles protocol, the layoutreturn should specify an
array of errors in the following format:

struct ff_ioerr4 {
	offset4        ffie_offset;
	length4        ffie_length;
	stateid4       ffie_stateid;
	device_error4  ffie_errors<>;
};

This patch fixes up the code to ensure that our ffie_errors is indeed
encoded as an array (albeit with only a single entry).

Reported-by: Tom Haynes <thomas.haynes@primarydata.com>
Cc: stable@vger.kernel.org
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>