i40e: return immediately when failing to add fdir filter

Instead of setting err=true and checking this to determine when to free
the raw_packet near the end of the function, simply kfree and return
immediately. The resulting code is a bit cleaner and has one less
variable. This also resolves a subtle bug in the ipv4 case which could
fail to add the first filter and then never free the memory, resulting
in a small memory leak.

Change-ID: I7583aac033481dc794b4acaa14445059c8930ff1
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Avinash Dayanand <avinash.dayanand@intel.com>
Reviewed-by: Alan Brady <alan.brady@intel.com>
Reviewed-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>