mlx4_core: Remove kfree() in mlx4_mr_alloc() error flow

mlx4_mr_alloc() doesn't actually allocate mr (it just initializes the
pointer that the caller passes in), so it shouldn't free it if an
error occurs.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>