mm: introduce lock_vma_under_rcu to be used from arch-specific code

mm: introduce lock_vma_under_rcu to be used from arch-specific code

Introduce lock_vma_under_rcu function to lookup and lock a VMA during
page fault handling. When VMA is not found, can't be locked or changes
after being locked, the function returns NULL. The lookup is performed
under RCU protection to prevent the found VMA from being destroyed before
the VMA lock is acquired. VMA lock statistics are updated according to
the results.
For now only anonymous VMAs can be searched this way. In other cases the
function returns NULL.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>