IB/mlx5: Add helper mlx5_ib_post_send_wait

Clean up the following common code (to post a list of work requests to the
send queue of the specified QP) at various places and add a helper function
'mlx5_ib_post_send_wait' to implement the same.

 - Initialize 'mlx5_ib_umr_context' on stack
 - Assign "mlx5_umr_wr:wr:wr_cqe to umr_context.cqe
 - Acquire the semaphore
 - call ib_post_send with a single ib_send_wr
 - wait_for_completion()
 - Check for umr_context.status
 - Release the semaphore

Signed-off-by: Binoy Jayan <binoy.jayan@linaro.org>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>