net: core: Fix IPv6 RPL extension parsing

RPL extension header verify function takes input parameter as
metadata 'buf' and start reading from it. Instead it should start
reading from fragment (buf->frags). Also API should return
fragment and offset where caller can continue reading it.

Change-Id: I80612144aed55aa2dcf9eaab1f4408e10efdda31
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>