ipsec: fix trailing data overflow

ipsec: fix trailing data overflow

IPsec writes trailing data at the end of the buffer without checking
if there is enough space. If the packet length equals buffer size this
leads to rewiting of the next buffer header in the pool.

Type: fix

Change-Id: Iceb27bb724c7243863a4b532aad0808051b7d74c
Signed-off-by: Filip Tehlar <ftehlar@cisco.com>