Host crash via deferred-lock rollback dereference

NULL-pointer dereference in the ksmbd SMB2 lock handler in Linux 5.15 through 7.2.5 allows remote authenticated users to crash the host. smb2_lock() adds a deferred ksmbd_lock to rollback_list before asynchronous work is allocated and registered, so setup failure makes rollback dereference an uninitialized conn pointer and frees a file_lock still linked in VFS state. Reachability requires an authenticated SMB lock request, VFS lock deferral, and asynchronous setup failure; the resulting fault can interrupt the entire host.