drivers: rmnet_perf: Use segmented length in mismatch checking

For any packets that have been previously segmented as a result of RSB/RSC,
the IP length in the header will not be correct in the descriptor case as we
have not yet created new headers. In this case, we need to calculate the
length of the segmented IP packet when checking for mismatches with the QMAP
length.

Change-Id: Iede7418b9245d1c4935755839f0b794588581332
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>