mm: handle faults that merely update the accessed bit under the VMA lock

Move FAULT_FLAG_VMA_LOCK check out of handle_pte_fault().  This should
have a significant performance improvement for mmaped files.  Write faults
(on read-only shared pages) still take the mmap lock as we do not want to
audit all the implementations of ->pfn_mkwrite() and ->page_mkwrite(). 
However write-faults on private mappings are handled under the VMA lock.

[[email protected]: address "suspicious RCU usage" warning]
  Link: https://lkml.kernel.org/r/[email protected]
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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