examples/ipsec-secgw: allow larger burst size for vectors

examples/ipsec-secgw: allow larger burst size for vectors

Allow larger burst size of vector event mode instead of restricting
to 32. Also restructure traffic type struct to have num pkts first
so that it is always in first cacheline. Also cache align
traffic type struct. Since MAX_PKT_BURST is not used by
vector event mode worker, define another macro for its burst
size so that poll mode perf is not effected.

Signed-off-by: Nithin Dabilpuram <ndabilpuram@marvell.com>
Acked-by: Akhil Goyal <gakhil@marvell.com>