net: add a per-cpu counter for the number of frames coalesced in GRO

A low cost method of determining GRO statistics is required. This
change introduces a new counter which tracks whenever GRO coalesces
ingress packets. The counter is per-CPU and exposed in
/proc/net/softnet_stat as the last column of data. No user space
impact is expected as a result of this change. However, this change
should be reverted if legacy tools have problems with the new column
in softnet_stat.

CRs-Fixed: 2333588
Change-Id: I05965c0cb150947935d5977884cc4d583b37131d
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>