smb3: use SMB2_SIGNATURE_SIZE define

It clarifies the code slightly to use SMB2_SIGNATURE_SIZE
define rather than 16.
Suggested-by: Henning Schild <henning.schild@siemens.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/smb2transport.c

@@ -602,7 +602,7 @@ int
 smb2_verify_signature(struct smb_rqst *rqst, struct TCP_Server_Info *server)
 {
 	unsigned int rc;
-	char server_response_sig[16];
+	char server_response_sig[SMB2_SIGNATURE_SIZE];
 	struct smb2_sync_hdr *shdr =
 			(struct smb2_sync_hdr *)rqst->rq_iov[0].iov_base;