drivers: rmnet_perf: Update logic for nonlinear SKBs

Perf now has to handle nonlinear SKBs arriving over a new path from the
core RmNet driver. No deaggregation will be performed. Instead, perf's
job is to perform GRO on packets that (possibly) have GSO information
already present in the SKB.

Change-Id: I66d5b01f0b0f0c45a8efcf79dc90f3cc22bc76e5
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>