md/md-llbitmap: stop daemon timer rearm on destroy

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

md/md-llbitmap: stop daemon timer rearm on destroy

llbitmap_destroy() deletes pending_timer before flushing
md_llbitmap_io_wq. However, daemon_work can still be queued or running
after the timer has been deleted, and the daemon path can arm
pending_timer again when it finds dirty chunks that are not ready to
flush yet.

If that happens during teardown, pending_timer can remain armed after
llbitmap is freed and later dereference freed memory.

Add a BITMAP_SHUTDOWN bit to llbitmap->flags, set it before deleting
the timer, and make the timer and daemon paths stop queueing or rearming
work once teardown starts. Cancel daemon_work before flushing the shared
workqueue so no already queued daemon instance can race with the free.
Use timer_shutdown_sync() so a daemon instance that passed the shutdown
check before teardown cannot rearm the timer afterward.

BITMAP_SHUTDOWN is a runtime-only state. Mask it out when reading and
updating the llbitmap superblock so the shutdown state is never loaded
from disk or persisted to disk.