libdm: dm-user target creation takes an additional argument

Rather than using some implicit naming scheme, we've decided to just let
userspace pick whatever control device naming scheme it wants.  This requires
an additional argument to be passed to the kernel.

Test: cow_snapuserd_test
Bug: 168259959
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Change-Id: Ibf7f378b67abb6ca7457a9fdc9b8739e3d31bee3
Signed-off-by: Akilesh Kailash <akailash@google.com>