net: test: Fix printf format specifier in skb_segment kunit test

KUNIT_FAIL() accepts a printf-style format string, but previously did
not let gcc validate it with the __printf() attribute. The use of %lld
for the result of PTR_ERR() is not correct.

Instead, use %pe and pass the actual error pointer. printk() will format
it correctly (and give a symbolic name rather than a number if
available, which should make the output more readable, too).

Fixes: b3098d32 ("net: add skb_segment kunit test")
Signed-off-by: David Gow <davidgow@google.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>