staging: erofs: fix potential overflow in z_erofs_vle_normalaccess_readpage{,s}

As Dan reported in the LKP mailing list:
https://lists.01.org/pipermail/kbuild-all/2018-August/051558.html



New smatch warnings:
drivers/staging/erofs/unzip_vle.c:1359 z_erofs_vle_normalaccess_readpages() warn: should '()->index << 12' be a 64 bit type?

Old smatch warnings:
drivers/staging/erofs/unzip_vle.c:1322 z_erofs_vle_normalaccess_readpage() warn: should 'page->index << 12' be a 64 bit type?

It needs to cast variable's type to erofs_off_t before left shifting.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>