rmnet_core: Check for contiguous QMAP headers during deaggregation

After the introduction of scatter gather support, it's possible that the
QMAP headers may be split across pages. This patch adds proper handling
for the headers during deaggregation from the original SKB from the
physical device when building the descriptors used elsewhere in the rmnet
driver.

Change-Id: If62cf8b4993fe01b6f43e9f16f08c134d18437b8
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>