block/iscsi:use the flags in iscsi_open() prevent Clang warning

Clang static code analyzer show warning:
  block/iscsi.c:1920:9: warning: Value stored to 'flags' is never read
        flags &= ~BDRV_O_RDWR;
        ^        ~~~~~~~~~~~~

In iscsi_allocmap_init() only checks BDRV_O_NOCACHE, which
is the same in both of flags and bs->open_flags.
We can use the flags instead bs->open_flags to prevent Clang warning.
Reported-by: NEuler Robot <euler.robot@huawei.com>
Signed-off-by: NChen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Message-Id: <20200311032927.35092-1-kuhn.chenqun@huawei.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/iscsi.c

@@ -2002,7 +2002,7 @@ static int iscsi_open(BlockDriverState *bs, QDict *options, int flags,
         iscsilun->cluster_size = iscsilun->bl.opt_unmap_gran *
             iscsilun->block_size;
         if (iscsilun->lbprz) {
-            ret = iscsi_allocmap_init(iscsilun, bs->open_flags);
+            ret = iscsi_allocmap_init(iscsilun, flags);
         }
     }