x86/mm: Fix and document DEBUG_PAGEALLOC

It turns out that commit 5fce67641a3e ("x86/mm/pat: Don't gate
cpa_lock on debug_pagealloc_enabled()") was a little too quick to
remove the debug_pagealloc exception for cpa_lock.

Notably __kernel_map_pages() is used by the page-allocator from any
context the page-allocator itself is used, which violates the cpa_lock
rules.

Re-instate the exception, except make it specific to the
__kernel_map_pages() such that any other cpa() usage is still fully
serialized by cpa_lock. Also note that since cpa() should not be used
on memory that isn't allocated, the page-allocator locking and cpa are
infact mutually exclusive and all cpa usage in fully serialized.

Add a comment explaining this and other 'funnies' surrounding
DEBUG_PAGEALLOC, including how pgd_lock is not affected and the TLB
trickery.

Fixes: 5fce67641a3e ("x86/mm/pat: Don't gate cpa_lock on debug_pagealloc_enabled()")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260729111119.604452135@infradead.org

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