fm10k: Check tunnel header length in encap offload

fm10k supports up to 184 bytes of inner+outer headers. Add an initial
check to fail encap offload if these are too large.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Tested-by: Krishneil Singh <Krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>