smb/client: avoid dereferencing rdata=NULL in smb2_new_read_req()

This happens when called from SMB2_read() while using rdma
and reaching the rdma_readwrite_threshold.

Cc: stable@vger.kernel.org
Fixes: a6559cc1 ("cifs: split out smb3_use_rdma_offload() helper")
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Steve French <stfrench@microsoft.com>