rhashtable: replace rht_ptr_locked() with rht_assign_locked()

The only times rht_ptr_locked() is used, it is to store a new
value in a bucket-head.  This is the only time it makes sense
to use it too.  So replace it by a function which does the
whole task:  Sets the lock bit and assigns to a bucket head.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>