generic/pgtable: Introduce set_pte_safe()

Commit:

  f77084d9 "x86/mm/pat: Disable preemption around __flush_tlb_all()"

introduced a warning to capture cases __flush_tlb_all() is called without
pre-emption disabled. It triggers a false positive warning in the memory
hotplug path.

On investigation it was found that the __flush_tlb_all() calls are not
necessary. However, they are only "not necessary" in practice provided
the ptes are being initially populated from the !present state.

Introduce set_pte_safe() as a sanity check that the pte is being updated
in a way that does not require a TLB flush.

Forgive the macro, the availability of the various of set_pte() levels
is hit and miss across architectures.

[ mingo: Minor readability edits. ]

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/279dadae-9148-465c-7ec6-3f37e026c6c9@intel.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>