xfs: noalloc allocation groups

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.