RDMA: Fix double-free in srq creation error flow

The cited commit introduced a double-free of the srq buffer in the error
flow of procedure __uverbs_create_xsrq().

The problem is that ib_destroy_srq_user() called in the error flow also
frees the srq buffer.

Thus, if uverbs_response() fails in __uverbs_create_srq(), the srq buffer
will be freed twice.

Cc: <stable@vger.kernel.org>
Fixes: 68e326dea1 ("RDMA: Handle SRQ allocations by IB/core")
Link: https://lore.kernel.org/r/20190916071154.20383-5-leon@kernel.org
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/uverbs_cmd.c

@@ -3482,7 +3482,8 @@ static int __uverbs_create_xsrq(struct uverbs_attr_bundle *attrs,
 
 err_copy:
 	ib_destroy_srq_user(srq, uverbs_get_cleared_udata(attrs));
-
+	/* It was released in ib_destroy_srq_user */
+	srq = NULL;
 err_free:
 	kfree(srq);
 err_put: