selftests/livepatch: Replace hardcoded module name with variable in test-callbacks.sh

Replaced the hardcoded module name test_klp_callbacks_demo in the
pre_patch_callback log message with the variable $MOD_LIVEPATCH.

Signed-off-by: George Guo <guodongtai@kylinos.cn>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20241125112812.281018-2-dongtai.guo@linux.dev


Signed-off-by: Petr Mladek <pmladek@suse.com>