seperate user and kernel sg table handling

This is a preperation patch for the sapport of
the kernel's scatterelist and sg_table as the
SG implementation of choice in the kernel.

Change-Id: If05b63d18a936ca3c3cb968be59b39c01a49d98d
Signed-off-by: Shlomo Pongratz <shlomop@mellanox.com>