Impact: future-proof the split_large_page() function

Linus noticed that split_large_page() is not safe wrt. the
PAT bit: it is bit 12 on the 1GB and 2MB page table level
(_PAGE_BIT_PAT_LARGE), and it is bit 7 on the 4K page
table level (_PAGE_BIT_PAT).

Currently it is not a problem because we never set
_PAGE_BIT_PAT_LARGE on any of the large-page mappings - but
should this happen in the future the split_large_page() would
silently lift bit 12 into the lowlevel 4K pte and would start
corrupting the physical page frame offset. Not fun.

So add a debug warning, to make sure if something ever sets
the PAT bit then this function gets updated too.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>