smb3: workaround negprot bug in some Samba servers

Mount can now fail to older Samba servers due to a server
bug handling padding at the end of the last negotiate
context (negotiate contexts typically are rounded up to 8
bytes by adding padding if needed). This server bug can
be avoided by switching the order of negotiate contexts,
placing a negotiate context at the end that does not
require padding (prior to the recent netname context fix
this was the case on the client).

Fixes: 73130a7b ("smb3: fix empty netname context on secondary channels")
Reported-by: Julian Sikorski <belegdol@gmail.com>
Tested-by: Julian Sikorski <belegdol+github@gmail.com>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/smb2pdu.c

@@ -571,10 +571,6 @@ assemble_neg_contexts(struct smb2_negotiate_req *req,
 	*total_len += ctxt_len;
 	pneg_ctxt += ctxt_len;
 
-	build_posix_ctxt((struct smb2_posix_neg_context *)pneg_ctxt);
-	*total_len += sizeof(struct smb2_posix_neg_context);
-	pneg_ctxt += sizeof(struct smb2_posix_neg_context);
-
 	/*
 	 * secondary channels don't have the hostname field populated
 	 * use the hostname field in the primary channel instead
@@ -586,9 +582,14 @@ assemble_neg_contexts(struct smb2_negotiate_req *req,
 					      hostname);
 		*total_len += ctxt_len;
 		pneg_ctxt += ctxt_len;
-		neg_context_count = 4;
-	} else /* second channels do not have a hostname */
 		neg_context_count = 3;
+	} else
+		neg_context_count = 2;
+
+	build_posix_ctxt((struct smb2_posix_neg_context *)pneg_ctxt);
+	*total_len += sizeof(struct smb2_posix_neg_context);
+	pneg_ctxt += sizeof(struct smb2_posix_neg_context);
+	neg_context_count++;
 
 	if (server->compress_algorithm) {
 		build_compression_ctxt((struct smb2_compression_capabilities_context *)