[PKTGEN]: Fix double unlock of xfrm_state->lock

[PKTGEN]: Fix double unlock of xfrm_state->lock

The pktgen_output_ipsec() function can unlock this lock twice
due to merged error and plain paths. Remove one of the calls
to spin_unlock.

Other possible solution would be to place "return 0" right
after the first unlock, but at this place the err is known
to be 0, so these solutions are the same except for this one
makes the code shorter.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>