xfs: use per-RTG bitmaps and summaries

xfs: use per-RTG bitmaps and summaries

Allow RT allocation to scale by making the bitmap and summary inodes
per-RTG.  The 64-bit wide xfs_rtxnum_t becomes relative to the RTG and
now is the main data type used in the allocator - xfs_rgbno is left for
use with the rmap and refcount trees and can remain 32-bits as it never
exists for a pre-RTG file systems.

sb_rbmblocks and m_rsumlevels become per-RTG and we always allocate them
for full RTG, even when the last one doesn't use all blocks.  That
simplifies growfs a lot.

This needs a careful audit (and sparse loopback based tests) to verify
there are no overflows when converting to the xfs_rtxnum_t for
legacy file systems.  I'm almost sure we have issues with that
somewhere at the moment.

Signed-off-by: Christoph Hellwig <hch@lst.de>