mm: handle shared faults under the VMA lock

There are many implementations of ->fault and some of them depend on
mmap_lock being held.  All vm_ops that implement ->map_pages() end up
calling filemap_fault(), which I have audited to be sure it does not rely
on mmap_lock.  So (for now) key off ->map_pages existing as a flag to
indicate that it's safe to call ->fault while only holding the vma lock.

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

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