In case of error, the function read_mapping_page() returns
ERR_PTR() not NULL. The NULL test in the return value check
should be replaced with IS_ERR().

Fixes: fe7c2423 ("erofs: use read_mapping_page instead of sb_bread")
Reviewed-by: NGao Xiang <gaoxiang25@huawei.com>
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20190918083033.47780-1-weiyongjun1@huawei.comSigned-off-by: NGao Xiang <gaoxiang25@huawei.com>