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.
Signed-off-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v2)
Reviewed-by: NJason Gunthorpe <jgg@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/394773/