smb: client: fix shared DFS root mounts with different prefixes

[ Upstream commit 3ae872de ]

When having two DFS root mounts that are connected to same namespace,
same mount options but different prefix paths, we can't really use the
shared @server->origin_fullpath when chasing DFS links in them.

Move the origin_fullpath field to cifs_tcon structure so when having
shared DFS root mounts with different prefix paths, and we need to
chase any DFS links, dfs_get_automount_devname() will pick up the
correct full path out of the @tcon that will be used for the new
mount.

Before patch

  mount.cifs //dom/dfs/dir /mnt/1 -o ...
  mount.cifs //dom/dfs /mnt/2 -o ...
  # shared server, ses, tcon
  # server: origin_fullpath=//dom/dfs/dir

  # @server->origin_fullpath + '/dir/link1'
  $ ls /mnt/2/dir/link1
  ls: cannot open directory '/mnt/2/dir/link1': No such file or directory

After patch

  mount.cifs //dom/dfs/dir /mnt/1 -o ...
  mount.cifs //dom/dfs /mnt/2 -o ...
  # shared server & ses
  # tcon_1: origin_fullpath=//dom/dfs/dir
  # tcon_2: origin_fullpath=//dom/dfs

  # @tcon_2->origin_fullpath + '/dir/link1'
  $ ls /mnt/2/dir/link1
  dir0  dir1  dir10  dir3  dir5  dir6  dir7  dir9  target2_file.txt  tsub

Fixes: 8e355415 ("cifs: fix sharing of DFS connections")
Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>