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from TCP to UDP and the rsize/wsize/readdirsize is greater than NFS_MAXDGRAMDATA, it is possible for a thread doing an I/O RPC to get stuck repeatedly doing retries. This happens because the RPC will use a resize/wsize/readdirsize that won't work for UDP and, as such, it will keep failing indefinitely. This patch returns an error for this case, to avoid the problem. A discussion on freebsd-fs@ seemed to indicate that returning an error was preferable to silently ignoring the "udp"/"mntudp" option. This problem was discovered while investigating a problem reported by pjd@ via email. MFC after: 2 weeks