ksmbd: safely drain sessions during logoff

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

ksmbd: safely drain sessions during logoff

SMB3 multichannel allows requests for one session to run on multiple
connections. Wait for all channels bound to a session before freeing
shared session objects.

A deferred byte-range lock remains counted as a running request and only
wakes when its file closes. Wake blocked locks during the drain without
unpublishing or modifying their file objects. Synchronous CANCEL requests
must invoke their cancellation callback to wake pending operations, while
CHANGE_NOTIFY completion remains specific to the asynchronous path.

Serialize session teardown with channel registration and previous-session
cleanup, and use atomic work-state transitions so LOGOFF, CANCEL, and
connection teardown invoke cancellation callbacks only once.