net: ipv6: Fix pending buf leak when NA is received

The pending buf needs to be freed after it has been sent
to the network. We took the ref when pending buf was saved,
now we need to unref it when it is about to be sent.

Change-Id: I1e429969895700000a8aa124bd645db2d52d036c
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>