ksmbd: hold a connection reference for each session channel

In the Linux kernel, the following vulnerability has been resolved:

ksmbd: hold a connection reference for each session channel

ksmbd_session_unregister() removes a session from the global and per-
connection tables before dropping the table reference. A concurrent
reference holder can keep the session and its channel list alive after
those removals.

Connection cleanup can no longer find the unpublished session to remove its
channels. Since a channel only stores a raw conn pointer, the connection
can be freed while another session reference holder is still accessing it,
for example during SMB3.1.1 binding or a procfs read.

Take a connection reference when registering a channel and release it on
insertion failure, channel removal, or final session destruction. Keep the
channels and signing keys until the session is destroyed so a response can
still be signed after the session has been unregistered.