smb: client: ensure to try all targets when finding nested links

With current implementation, when a nested DFS link is found during
mount(2), the client follows the referral and then try to connect to
all of its targets. If all targets failed, the client bails out
rather than retrying remaining targets from previous referral.

Fix this by stacking all referrals and targets so the client can retry
remaining targets from previous referrals in case all targets of
current referral have failed.

Thanks to samba, this can be easily tested like below

* Run the following under dfs folder in samba server

$ ln -s "msdfs:srv\\bad-share" link1
$ ln -s "msdfs:srv\\dfs\\link1,srv\\good-share" link0

* Before patch

$ mount.cifs //srv/dfs/link0 /mnt -o ...
mount error(2): No such file or directory
Refer to the mount.cifs(8) manual page (e.g. man mount.cifs)...

* After patch

$ mount.cifs //srv/dfs/link0 /mnt -o ...
# ls /mnt
bar fileshare1 sub

Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

smb: client: introduce DFS_CACHE_TGT_LIST()

Add new helper which declares and initialises target list of a DFS
referral rather having to do both separately.

No functional changes.

Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

smb: move client and server files to common directory fs/smb

Move CIFS/SMB3 related client and server files (cifs.ko and ksmbd.ko
and helper modules) to new fs/smb subdirectory:

fs/cifs --> fs/smb/client
fs/ksmbd --> fs/smb/server
fs/smbfs_common --> fs/smb/common

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>