i40e: Fix missing rtnl locking when setting up pf switch

A recent change that made i40e use new udp_tunnel infrastructure
uses a method that expects to be called under rtnl lock.

However, not all codepaths do the lock prior to calling
i40e_setup_pf_switch.

Fix that by adding additional rtnl locking and unlocking.

Fixes: 40a98cb6 ("i40e: convert to new udp_tunnel infrastructure")
Signed-off-by: Jan Sokolowski <jan.sokolowski@intel.com>
Signed-off-by: Mateusz Palczewski <mateusz.palczewski@intel.com>
Tested-by: Tony Brelinski <tonyx.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>