kernel: Replace deprecated SUBDIRS with M in Makefile

[ReeseWang]

Please sign (check) the below before submitting the Pull Request:

• [x] I have signed the ntop Contributor License Agreement at https://github.com/ntop/legal/blob/main/individual-contributor-licence-agreement.md
• [x] I have updated the documentation (in doc/) to reflect the changes made (if applicable)

Describe changes:

The use of SUBDIRS to specify the location of external module source files has been deprecated since Linux kernel version 5.3. It caused unintended file deletion in my Linux 6.8.1 source tree. The recommended replacement is to use M instead.

This commit updates the Makefile to use M=$(HERE) instead of SUBDIRS=$(HERE) in the 'clean' target. This change ensures compatibility with newer Linux kernels and avoids potential issues during the build process, such as the unintended deletion of files.