net: qualcomm: rmnet: Introduce descriptor framework

Introduces a generic packet descriptor framework to allow RmNet to avoid
allocating intermediate SKBs. Instead, SKBs for the incoming packets will
allocated only when all possible forms of coalescing have been performed
to avoid unnecessary allocs and frees.

Change-Id: Ic3e4c9f174f2129faa2169c3b3f449becd2b5bdf
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>