ksmbd: use list_first_entry_or_null for opinfo_get_list()

The list_first_entry() macro never returns NULL.  If the list is
empty then it returns an invalid pointer.  Use list_first_entry_or_null()
to check if the list is empty.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202505080231.7OXwq4Te-lkp@intel.com/


Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>