x86/mm/pti: Remove dead logic in pti_user_pagetable_walk*()

The following code contains dead logic:

 162 if (pgd_none(*pgd)) {
 163         unsigned long new_p4d_page = __get_free_page(gfp);
 164         if (!new_p4d_page)
 165                 return NULL;
 166
 167         if (pgd_none(*pgd)) {
 168                 set_pgd(pgd, __pgd(_KERNPG_TABLE | __pa(new_p4d_page)));
 169                 new_p4d_page = 0;
 170         }
 171         if (new_p4d_page)
 172                 free_page(new_p4d_page);
 173 }

There can't be any difference between two pgd_none(*pgd) at L162 and L167,
so it's always false at L171.

Dave Hansen explained:

 Yes, the double-test was part of an optimization where we attempted to
 avoid using a global spinlock in the fork() path.  We would check for
 unallocated mid-level page tables without the lock.  The lock was only
 taken when we needed to *make* an entry to avoid collisions.
 
 Now that it is all single-threaded, there is no chance of a collision,
 no need for a lock, and no need for the re-check.

As all these functions are only called during init, mark them __init as
well.

Fixes: 03f4424f348e ("x86/mm/pti: Add functions to clone kernel PMDs")
Signed-off-by: Jike Song <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Alan Cox <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Tom Lendacky <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: Jiri Koshina <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Andi Lutomirski <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Greg KH <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Paul Turner <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]

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