dm io:ctl remove vmalloc void cast

In drivers/md/dm-ioctl.c::copy_params() there's a call to vmalloc()
where we currently cast the return value, but that's pretty pointless
given that vmalloc() returns "void *".

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>