xfs: start creating infrastructure for health monitoring

xfs: start creating infrastructure for health monitoring

Start creating helper functions and infrastructure to pass filesystem
health events to a health monitoring file.  Since this is an
administrative interface, we only support a single health monitor
process per filesystem, so we don't need to use anything fancy such as
notifier chains (== tons of indirect calls).

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>