smb: client: remove separate empty_packet_queue

There's no need to maintain two lists, we can just
have a single list of receive buffers, which are free to use.

It just added unneeded complexity and resulted in
ib_dma_unmap_single() not being called from recv_done()
for empty keepalive packets.

Cc: Steve French <smfrench@gmail.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Long Li <longli@microsoft.com>
Cc: linux-cifs@vger.kernel.org
Cc: samba-technical@lists.samba.org
Fixes: f198186a ("CIFS: SMBD: Establish SMB Direct connection")
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Steve French <stfrench@microsoft.com>