nfsd4: fix sessions slotid wraparound logic

From RFC 5661 2.10.6.1: "If the previous sequence ID was 0xFFFFFFFF,
then the next request for the slot MUST have the sequence ID set to
zero."

While we're there, delete some redundant comments.
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -1575,16 +1575,11 @@ check_slot_seqid(u32 seqid, u32 slot_seqid, int slot_inuse)
 		else
 			return nfserr_seq_misordered;
 	}
-	/* Normal */
+	/* Note unsigned 32-bit arithmetic handles wraparound: */
 	if (likely(seqid == slot_seqid + 1))
 		return nfs_ok;
-	/* Replay */
 	if (seqid == slot_seqid)
 		return nfserr_replay_cache;
-	/* Wraparound */
-	if (seqid == 1 && (slot_seqid + 1) == 0)
-		return nfs_ok;
-	/* Misordered replay or misordered new request */
 	return nfserr_seq_misordered;
 }