IB/mlx4: Fix leaks in __mlx4_ib_modify_qp

Temporarily allocated struct mlx4_qp_context *context is leaked by
several error paths.  The patch takes advantage of the return value
'err' being preinitialized to -EINVAL.

Spotted by Coverity (CID 1768).

Signed-off-by: Florin Malita <fmalita@gmail.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>