smb:client: smb: client: Add reverse mapping from tcon to superblocks

Currently, when a SMB connection is reset and renegotiated with the
server, there's no way to update all related mount points with new
negotiated sizes. This is because while superblocks (cifs_sb_info)
maintain references to tree connections (tcon) through tcon_link
structures, there is no reverse mapping from a tcon back to all the
superblocks using it.

This patch adds a bidirectional relationship between tcon and
cifs_sb_info structures by:

1. Adding a cifs_sb_list to tcon structure with appropriate locking
2. Adding tcon_sb_link to cifs_sb_info to join the list
3. Managing the list entries during mount and umount operations

The bidirectional relationship enables future functionality to locate and
update all superblocks connected to a specific tree connection, such as:

- Updating negotiated parameters after reconnection
- Efficiently notifying all affected mounts of capability changes

This is the first part of a series to improve connection resilience
by keeping all mount parameters in sync with server capabilities
after reconnection.

Signed-off-by: Wang Zhaolong <[email protected]>
Signed-off-by: Steve French <[email protected]>

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