rmnet_data : Changes to support Scatter Gather and GSO.

This patch ensures to skip padding(to align length
to word boundaries) for outgoing  non linear skbs.

This patch also adds a new ioctl interface
RMNET_IOCTL_GET_SG_SUPPORT  to query the physical
network devices for Scatter Gather support.

Change-Id: I9788d75c249ab2dac5b598dad131c0692ed84e4d
Acked-by: Abhishek Chauhan <abchauha@qti.qualcomm.com>
Signed-off-by: Ravinder Konka <rkonka@codeaurora.org>