Avoid using `try_module_get(THIS_MODULE)`

[jserv]

There are various deprecated functions/macros used in LKMPG. For example, try_module_get(THIS_MODULE) is regarded as unsafe.

See https://stackoverflow.com/questions/1741415/linux-kernel-modules-when-to-use-try-module-get-module-put

[ashevche]

I believe the book should state for which kernel versions it's applicable.