mm: delay the check for a NULL anon_vma

Instead of checking the anon_vma early in the fault path where all page
faults pay the cost, delay it until we know we're going to need the
anon_vma to be filled in.  This will have a slight negative effect on the
first fault in an anonymous VMA, but it shortens every other page fault. 
It also makes the code slightly cleaner as the anon and file backed fault
handling look more similar.

The Intel kernel test bot reports a 3x improvement in vm-scalability
throughput with the small-allocs-mt test.  This is clearly an extreme
situation that won't be replicated in any real-world workload, but it's a
nice win.

https://lore.kernel.org/all/[email protected]/

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Reviewed-by: Suren Baghdasaryan <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jann Horn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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