RDMA/siw: Fix post_recv QP state locking

Do not release qp state lock if not previously acquired.

Fixes: cf049bb3

 ("RDMA/siw: Fix SQ/RQ drain logic")
Link: https://lore.kernel.org/r/20191025142903.20625-1-bmt@zurich.ibm.com
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bernard Metzler <bmt@zurich.ibm.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>