x86/mm/pat: Use pr_warn() for early W^X warnings

Nathan Chancellor reports the following warning:

  CPA detected W^X violation: 8000000000000123 -> 0000000000000123 range: 0xffffffffc0400000 - 0xffffffffc0400fff PFN 100e00

from its_alloc() doing set_memory_x() during early retpoline setup.

The warning is not factually wrong but it is unproductive. First, the RWX
permission is temporary and fixed up by execmem_restore_rox(). Second,
at the time of the warning, all kernel text is mapped RWX. So there is not
even a transient actual security issue.

The right way to fix this up is to relax RWX detection during boot but
make a final verification pass over all kernel page tables before running
userspace.  There is code to do that today (debug_checkwx()) but it
must be enabled explicitly in Kconfig.

For now, retain the warning even if it's unproductive, but make it
scream less loudly: use pr_warn_once() rather than WARN_ONCE().

Remove the warning once debug_checkwx() behavior becomes unconditional.

Closes: https://lore.kernel.org/all/20260905044253.GA3816371@ax162
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://patch.msgid.link/20260913-fixes-verify-rwx-v3-1-5e1d6a08bd02@kernel.org

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