Fix error recovery path in __gnttab_map_grant_ref

Fix error recovery path in __gnttab_map_grant_ref

In file grant_table.c function __gnttab_map_grant_ref, if
__get_paged_frame failed, the effect of _set_status  previously
called should be rollback, so the flag GTF_reading and _GTF_writing
will be recovered.

Signed-off-by: Haoyu Zhang <haoyu.zhang@huawei.com>
Signed-off-by: Liang Wang <hzwangliang.wang@huawei.com>
Acked-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Committed-by: Keir Fraser <keir@xen.org>