mem: support unmapping pages at runtime

This isn't used anywhere yet, but the support is now there. Also,
adding cleanup to allocation procedures, so that if we fail to
allocate everything we asked for, we can free all of it back.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Tested-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>
Tested-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Tested-by: Gowrishankar Muthukrishnan <gowrishankar.m@linux.vnet.ibm.com>