x86: avoid theoretical vmalloc fault loop

Ajith Kumar noticed:

 I was going through the vmalloc fault handling for x86_64 and am unclear
 about the following lines in the vmalloc_fault() function.

 pgd = pgd_offset(current->mm ?: &init_mm, address);
 pgd_ref = pgd_offset_k(address);

 Here the intention is to get the pgd corresponding to the current process
 and sync it up with the pgd in init_mm(obtained from pgd_offset_k).
 However, for kernel threads current->mm is NULL and hence pgd =
 pgd_offset(init_mm, address) = pgd_ref which means the fault handler
 returns without setting the pgd entry in the MM structure in the context
 of which the kernel thread has faulted.  This could lead to never-ending
 faults and busy looping of kernel threads like pdflush.  So, shouldn't the
 pgd = pgd_offset(current->mm ?: &init_mm, address); be pgd =
 pgd_offset(current->active_mm ?: &init_mm, address);

We can use active_mm unconditionally because it should be always set.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>