xfs: force rebuilding of metadata [v26.1]

This patchset adds a new IFLAG to the scrub ioctl so that userspace can
force a rebuild of an otherwise consistent piece of metadata.  This will
eventually enable the use of online repair to relocate metadata during a
filesystem reorganization (e.g. shrink).  For now, it facilitates stress
testing of online repair without needing the debugging knobs to be
enabled.

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

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