xfs: account for log space when formatting new AGs

xfs: account for log space when formatting new AGs

Source kernel commit: f327a00745fffd9159d54b442cb75c0266fb89d6

When we're writing out a fresh new AG, make sure that we don't list an
internal log as free and that we create the rmap for the region.  growfs
never does this, but we will need it when we hook up mkfs.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>