x86/mm/pat: Don't gate cpa_lock on debug_pagealloc_enabled()

The splitting and merging of kernel page table mappings between small and
large is protected by cpa_lock. The merging is relatively new but the
splitting is ancient.

The splitting has a locking optimization: since DEBUG_PAGEALLOC forces all
mappings to 4k, there are no large pages to split. So the code that *might*
cause a split can just skip the locking (and a few other things).

This is entertaining, but it adds complexity and makes for weird locking
rules. Plus it's all for a debugging feature which makes the kernel super
slow in the first place. Optimizing something which is already super slow
and not used in production is not the best way to spend our complexity
budget.

Stop gating cpa_lock on debug_pagealloc_enabled() to simplify the code
and the locking rules.

[ dhansen: flesh out changelog ]

Suggested-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://patch.msgid.link/20260715144519.934289-1-rppt@kernel.org
Link: https://lore.kernel.org/all/aab44f08-89f8-47fe-bee4-0ab6b25968c6@intel.com/

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