xfs: live health monitoring of filesystems [RFC 52/54]

This patchset builds off of Kent Overstreet's thread_with_file code to
deliver live information about filesystem health events to userspace.
This is done by creating a twf file and hooking internal operations so
that the event information can be queued to the twf without stalling the
kernel if the twf client program is nonresponsive.  This is a private
ioctl, so events are expressed using simple json objects so that we can
enrich the output later on without having to rev a ton of C structs.

In userspace, we create a new daemon program that will read the json
event objects and initiate repairs automatically.  This daemon is
managed entirely by systemd and will not block unmounting of the
filesystem unless repairs are ongoing.  It is autostarted via some
horrible udev rules.

This has been running on the djcloud for months with no problems.  Enjoy!

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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