IB/mlx4: Actually return L_Key and R_Key for fast register MRs

Initialize the L_Key and R_Key for memory regions returned from
mlx4_ib_alloc_fast_reg_mr().  Otherwise callers just get garbage for
the memory keys and can't do anything useful with these MRs.

Signed-off-by: Vladimir Sokolovsky <vlad@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>