xfs: noalloc allocation groups [RFC 44/54]

This series creates a new NOALLOC flag for allocation groups that causes
the block and inode allocators to look elsewhere when trying to
allocate resources.  This is either the first part of a patchset to
implement online shrinking (set noalloc on the last AGs, run fsr to move
the files and directories) or freeze-free rmapbt rebuilding (set
noalloc to prevent creation of new mappings, then hook deletion of old
mappings).  This is still totally a research project.

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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