ip_frag: increase default maximum of fragments

ip_frag: increase default maximum of fragments

Increase default value for config parameter RTE_LIBRTE_IP_FRAG_MAX_FRAG
from 4 to 8. This parameter controls maximum number of fragments per
packet in ip reassembly table. Increasing this value from 4 to 8 will
allow users to cover common case with jumbo packet size of 9KB and
fragments with default frame size (1500B).
As RTE_LIBRTE_IP_FRAG_MAX_FRAG is used in definition of public
structure (struct rte_ip_frag_death_row), this is an ABI change.

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>