mm: handle PUD faults under the VMA lock

Postpone checking the VMA_LOCK flag until we've attempted to handle faults
on PUDs.  There's a mild upside to this patch in that we'll allocate the
page tables while under the VMA lock rather than the mmap lock, reducing
the hold time on the mmap lock, since the retry will find the page tables
already populated.  The real purpose here is to make a commit that shows
we don't call ->huge_fault under the VMA lock.  We do now handle setting
the accessed bit on a PUD fault under the VMA lock, but that doesn't seem
likely to be a measurable difference.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Cc: Arjun Roy <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Punit Agrawal <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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