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().

PR: 260012

(cherry picked from commit bdd57cbb1bdafcf2ebffa73c52f0fffc9410ea7b)