NFSD: Avoid calling OPDESC() with ops->opnum == OP_ILLEGAL

In the Linux kernel, the following vulnerability has been resolved:

NFSD: Avoid calling OPDESC() with ops->opnum == OP_ILLEGAL

OPDESC() simply indexes into nfsd4_ops[] by the op's operation number, without range checking that value. It assumes callers are careful to avoid calling it with an out-of-bounds opnum value.

nfsd4_decode_compound() is not so careful, and can invoke OPDESC() with opnum set to OP_ILLEGAL, which is 10044 -- well beyond the end of nfsd4_ops[].