x86/mm: Validate kernel_physical_mapping_init() PTE population

The usage of __flush_tlb_all() in the kernel_physical_mapping_init()
path is not necessary. In general flushing the TLB is not required when
updating an entry from the !present state. However, to give confidence
in the future removal of TLB flushing in this path, use the new
set_pte_safe() family of helpers to assert that the !present assumption
is true in this path.

[ 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: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
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: http://lkml.kernel.org/r/154395944177.32119.8524957429632012270.stgit@dwillia2-desk3.amr.corp.intel.com


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