smb: client: fix DFS interlink failover

[ Upstream commit 4f42a8b5 ]

The DFS interlinks point to different DFS namespaces so make sure to
use the correct DFS root server to chase any DFS links under it by
storing the SMB session in dfs_ref_walk structure and then using it on
every referral walk.

Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Stable-dep-of: 74ebd021 ("cifs: all initializations for tcon should happen in tcon_info_alloc")
Signed-off-by: Sasha Levin <sashal@kernel.org>