packet pool: fix ext_pkt cleaning

The field ext_pkt was cleaned before calling the release function.
The result was that IPS mode such as the one of AF_PACKET were not
working anymore because they were not able to send the data which
were initially pointed by ext_pkt.

This patch moves the ext_pkt cleaning to the cleaning macro. This
ensures that the cleaning is done for allocated and pool packets.

Redmine ticket: https://redmine.openinfosecfoundation.org/issues/1259

PR builds:
- PR build: https://buildbot.openinfosecfoundation.org/builders/regit/builds/9
- PR pcaps: https://buildbot.openinfosecfoundation.org/builders/regit-pcap/builds/7