mem: add contig walk function

mem: add contig walk function

This function is meant to walk over first segment of each
VA-contiguous group of memsegs.

For future users of this function, this is done so that
there is less dependency on internals of mem API and less
noise later change sets.

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>