mm: Delay the check for a NULL anon_vma

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.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>