rmnet_perf: Validate checksum for all incoming packets

Previously, checksum validation was performed only for TCP
packets that were being inserted into already-existing flows.
Now, all incoming packets will undergo QMAPv4 checksum validation.
If a packet fails, it is immediately flushed out to stack, otherwise
processing is allowed to continue.

Change-Id: I171b9a9462305c74faf7f2f3ecdaac7a6129894c
CRs-Fixed: 2344359
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>