net/mlx5: Only create VEPA flow table when in VEPA mode

Currently, when VFs are created, two flow tables are added for the eswitch:
the "fdb" table, which contains rules for each VF and the "vepa_fdb" table.
In the default VEB mode, the vepa_fdb table is empty. When switching to
VEPA mode, flow steering rules are added to vepa_fdb. Even though the
vepa_fdb table is empty in VEB mode, its presence adds some cost to packet
processing. In some workloads, this leads to drops which are reported by
the rx_discards_phy ethtool counter.

In order to improve performance, only create vepa_fdb when in VEPA mode.

Tests were done on a ConnectX-6 Lx adapter forwarding 64B packets between
both ports using dpdk-testpmd. Numbers are Rx-pps for each port, as
reported by testpmd.

Without changes:
traffic to unknown mac
	testpmd on PF
		numvfs=0,0
			35257998,35264499
		numvfs=1,1
			24590124,24590888
	testpmd on VF with numvfs=1,1
		20434338,20434887
traffic to VF mac
	testpmd on VF with numvfs=1,1
		30341014,30340749

With changes:
traffic to unknown mac
	testpmd on PF
		numvfs=0,0
			35404361,35383378
		numvfs=1,1
			29801247,29790757
	testpmd on VF with numvfs=1,1
		24310435,24309084
traffic to VF mac
	testpmd on VF with numvfs=1,1
		34811436,34781706

Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>