rmnet_perf: Manually parse TCP options

tcp_parse_options() requires that skb_transport_header is set
to a valid value since it uses the tcp_hdr() function. That
is not the case for our SKBs, so we must parse the header
manually for the timestamp.

CRs-Fixed: 2328576
Change-Id: Ifce1aeb07497ae3c89e18d0659631e3bdb377531
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>