ocfs2: Grow discontig block groups in one transaction.

ocfs2: Grow discontig block groups in one transaction.

Rather than extending the transaction every time we add an extent to a
discontiguous block group, we grab enough credits to fill the extent
list up front.  This means we can free the bits in the same transaction
if we end up not getting enough space.

Signed-off-by: Joel Becker <joel.becker@oracle.com>