uprobes: Kill uprobe->copy_mutex

Now that ->register_rwsem is safe under ->mmap_sem we can kill
->copy_mutex and abuse down_write(&uprobe->consumer_rwsem).

This makes prepare_uprobe() even more ugly, but we should kill
it anyway.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>