nfsd: never defer requests during idmap lookup

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

nfsd: never defer requests during idmap lookup

During v4 request compound arg decoding, some ops (e.g. SETATTR)

can trigger idmap lookup upcalls. When those upcall responses get

delayed beyond the allowed time limit, cache_check() will mark the

request for deferral and cause it to be dropped.

This prevents nfs4svc_encode_compoundres from being executed, and

thus the session slot flag NFSD4_SLOT_INUSE never gets cleared.

Subsequent client requests will fail with NFSERR_JUKEBOX, given

that the slot will be marked as in-use, making the SEQUENCE op

fail.

Fix this by making sure that the RQ_USEDEFERRAL flag is always

clear during nfs4svc_decode_compoundargs(), since no v4 request

should ever be deferred.