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[].

Reported-by: Jeff Layton <jlayton@kernel.org>
Fixes: f4f9ef4a

 ("nfsd4: opdesc will be useful outside nfs4proc.c")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>