Enabling page_poison=1 together with init_on_alloc=1 or init_on_free=1 produces
a warning in dmesg that page_poison takes precendence. However, as these
warnings are printed in early_param handlers for init_on_alloc/free, they are
not printed if page_poison is enabled later on the command line (handlers are
called in the order of their parameters), or when init_on_alloc/free is always
enabled by the respective config option - before the page_poison early param
handler is called, it is not considered to be enabled. This is inconsistent.
We can remove the dependency on order by making the init_on_* parameters only
set a boolean variable, and postponing the evaluation after all early params
have been processed. Introduce a new init_mem_debugging() function for that,
and move the related debug_pagealloc processing there as well.
As a result init_mem_debugging() knows always accurately if init_on_* and/or
page_poison options were enabled. Thus we can also optimize want_init_on_alloc()
and want_init_on_free(). We don't need to check page_poisoning_enabled() there,
we can instead not enable the init_on_* tracepoint at all, if page poisoning is
enabled. This results in a simpler and more effective code.