migration/rdma: Fix save_page method to fail on polling error

qemu_rdma_save_page() reports polling error with error_report(), then
succeeds anyway.  This is because the variable holding the polling
status *shadows* the variable the function returns.  The latter
remains zero.

Broken since day one, and duplicated more recently.

Fixes: 2da776db (rdma: core logic)
Fixes: b390afd8 (migration/rdma: Fix out of order wrid)
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Li Zhijian <lizhijian@fujitsu.com>
Message-ID: <20230921121312.1301864-2-armbru@redhat.com>