mem: add function to walk all memsegs

mem: add function to walk all memsegs

For code that might need to iterate over list of allocated
segments, using this API will make it more resilient to
internal API changes and will prevent copying the same
iteration code over and over again.

Additionally, down the line there will be locking implemented,
so users of this API will not need to care about locking
either.

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>