[PATCH] NFSD: check for negative "len" values in nfssvc_decode_writeargs()

[Dan Carpenter <dan.carpenter@xxxxxxxxxx>]

This code checks the upper bound of "len" but it needs to check for
negative values as well.

Fixes: a51b5b737a0b ("NFSD: Update the NFSv2 WRITE argument decoder to use struct xdr_stream")
Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
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