xfs: move extent busy tree initialization to xfs_initialize_perag

Move the per-AG busy extent tree initialization to the per-ag structure
initialization since we don't want online repair to leak the old tree.
We only deconstruct the tree at unmount time, so this should be safe.
This also enables us to eliminate the commented out initialization in
the xfsprogs libxfs.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>