xfs: set healthmon mount cookie before publishing it

xfs_ioc_health_monitor() allocates the healthmon object with kzalloc(), so
mount_cookie starts as zero.  zero is also DETACHED_MOUNT_COOKIE, but
xfs_healthmon_attach() publishes hm through mp->m_healthmon before storing
the mount cookie.  A concurrent reporter can therefore acquire hm from
xfs_healthmon_get(), see the detached cookie in xfs_healthmon_push(), and
drop the event even though the monitor is being attached.

This is reachable by racing XFS_IOC_HEALTH_MONITOR with an operation
that reports a health event, such as a filesystem shutdown.  Initialize
the mount cookie before making hm visible through the RCU pointer so
reporters always observe an attached monitor.

Found during a static-analysis-assisted audit: candidate sites were
surfaced by a Coccinelle rule, and the reader/writer paths were
confirmed by LLM-assisted (GLM-5.3) code inspection.  The issue was
not observed at runtime; it is identified by code inspection only.

Fixes: a48373e7d3 ("xfs: start creating infrastructure for health monitoring")
Cc: stable@vger.kernel.org
Signed-off-by: Binbin Deng <18983559317@163.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

fs/xfs/xfs_healthmon.c

@@ -114,8 +114,8 @@ xfs_healthmon_attach(
 	}
 
 	refcount_inc(&hm->ref);
-	rcu_assign_pointer(mp->m_healthmon, hm);
 	hm->mount_cookie = (uintptr_t)mp->m_super;
+	rcu_assign_pointer(mp->m_healthmon, hm);
 	spin_unlock(&xfs_healthmon_lock);
 
 	return 0;