mm: move vma locking out of vma_prepare and dup_anon_vma

vma_prepare() is currently the central place where vmas are being locked
before vma_complete() applies changes to them. While this is convenient,
it also obscures vma locking and makes it harder to follow the locking
rules. Move vma locking out of vma_prepare() and take vma locks
explicitly at the locations where vmas are being modified. Move vma
locking and replace it with an assertion inside dup_anon_vma() to further
clarify the locking pattern inside vma_merge().

Link: https://lkml.kernel.org/r/[email protected]
Suggested-by: Linus Torvalds <[email protected]>
Suggested-by: Liam R. Howlett <[email protected]>
Signed-off-by: Suren Baghdasaryan <[email protected]>
Cc: Jann Horn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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