kunit: skb: use "gfp" variable instead of hardcoding GFP_KERNEL

kunit: skb: use "gfp" variable instead of hardcoding GFP_KERNEL

The intent here was clearly to use the gfp variable flags instead of
hardcoding GFP_KERNEL.  All the callers pass GFP_KERNEL as the gfp
flags so this doesn't affect runtime.

Fixes: b3231d353a51 ("kunit: add a convenience allocation wrapper for SKBs")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>