rmnet: fix tx rmnet stats to work with UL aggregation

allows UL stats within rmnet to count packets which
are being aggregated.

Includes API changes to both tx and rx stats functions
to instead take skb lengths as arguments instead of
skb pointers to safeguard against cases where we might
have already freed the skb

CRs-Fixed: 2261938
Change-Id: I08d646f1641a5fe10bdcdf647c472d64f3f2551c
Signed-off-by: Conner Huff <chuff@codeaurora.org>