smb: smbdirect: introduce smbdirect_socket.workqueue

smb: smbdirect: introduce smbdirect_socket.workqueue

The client currently used a per socket workqueue
because it can block in a work function
waiting for credits.

So we use a per socket pointer in order to prepare
common code.

Cc: Steve French <smfrench@gmail.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Long Li <longli@microsoft.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Cc: linux-cifs@vger.kernel.org
Cc: samba-technical@lists.samba.org
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Steve French <stfrench@microsoft.com>