binder: rename alloc->buffer to vm_start

binder: rename alloc->buffer to vm_start

The alloc->buffer field in struct binder_alloc stores the starting
address of the mapped vma, rename this field to alloc->vm_start to
better reflect its purpose. It also avoids confusion with the binder
buffer concept, e.g. transaction->buffer.

No functional changes in this patch.

Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20241210143114.661252-7-cmllamas@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>