nfsd: Add checks for layout errors in LayoutReturn

For a LayoutReturn when using the Flexible File Layout,
error reports may be provided in the request.
Sanity check the size of these error reports and
check that they exist before calling nfsrv_flexlayouterr().

Reported by: rtm@lcs.mit.edu
Tested by: rtm@lcs.mit.edu
PR: 260012
MFC after: 2 weeks