kunit: skb: add gfp to kernel doc for kunit_zalloc_skb()

kunit: skb: add gfp to kernel doc for kunit_zalloc_skb()

Kuan-Wei Chiu pointed out that the kernel doc for kunit_zalloc_skb()
needs to include the @gfp information.  Add it.

Reported-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Closes: https://lore.kernel.org/all/Zy+VIXDPuU613fFd@visitorckw-System-Product-Name/
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>