nfsd: nfsd4_spo_must_allow() must check this is a v4 compound request

In the Linux kernel, the following vulnerability has been resolved: nfsd: nfsd4_spo_must_allow() must check this is a v4 compound request If the request being processed is not a v4 compound request, then examining the cstate can have undefined results. This patch adds a check that the rpc procedure being executed (rq_procinfo) is the NFSPROC4_COMPOUND procedure.