mm: introduce vma_set_file function v5

Add the new vma_set_file() function to allow changing
vma->vm_file with the necessary refcount dance.

v2: add more users of this.
v3: add missing EXPORT_SYMBOL, rebase on mmap cleanup,
    add comments why we drop the reference on two occasions.
v4: make it clear that changing an anonymous vma is illegal.
v5: move vma_set_file to mm/util.c

Signed-off-by: Christian König <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]> (v2)
Reviewed-by: Jason Gunthorpe <[email protected]>
Acked-by: Andrew Morton <[email protected]>
Link: https://patchwork.freedesktop.org/patch/399360/

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