JBD: Replace slab allocations with page allocations

JBD allocate memory for committed_data and frozen_data from slab. However
JBD should not pass slab pages down to the block layer. Use page allocator pages instead. This will also prepare JBD for the large blocksize patchset.
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>