net: add additional args validation in ping_common_sendmsg()

adding  validation of the len variable in ping_common_sendmsg()
to check if it is less than icmph_len which canleading to
an overflow issue.

Addressing issue reported under CVE-2016-8399.

Change-Id: I98f7b070b41312832b6a347ea1c11b9c700159a7
Signed-off-by: Ravi Kumar Siddojigari <rsiddoji@codeaurora.org>