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

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

[ Upstream commit 804d8e0a6e54427268790472781e03bc243f4ee3 ]

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: 57be5708fdbc ("nfsd4: opdesc will be useful outside nfs4proc.c")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>