xfs: stop reclaim before pushing AIL during unmount

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

xfs: stop reclaim before pushing AIL during unmount

The unmount sequence in xfs_unmount_flush_inodes() pushed the AIL while

background reclaim and inodegc are still running. This is broken

independently of any use-after-free issues - background reclaim and

inodegc should not be running while the AIL is being pushed during

unmount, as inodegc can dirty and insert inodes into the AIL during the

flush, and background reclaim can race to abort and free dirty inodes.

Reorder xfs_unmount_flush_inodes() to stop inodegc and cancel background

reclaim before pushing the AIL. Stop inodegc before cancelling

m_reclaim_work because the inodegc worker can re-queue m_reclaim_work

via xfs_inodegc_set_reclaimable.