xfs_healer: create a service to start the per-mount healer service

xfs_healer: create a service to start the per-mount healer service

Create a daemon to wait for xfs mount events via fsnotify and start up
the per-mount healer service.  It's important that we're running in the
same mount namespace as the mount, so we're a fanotify client to avoid
having to filter the mount namespaces ourselves.

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