block/nfs: fix nfs_client_open for filesize greater than 1TB

DIV_ROUND_UP(st.st_size, BDRV_SECTOR_SIZE) was overflowing ret (int) if
st.st_size is greater than 1TB.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Lieven <pl@kamp.de>
Message-id: 1511798407-31129-1-git-send-email-pl@kamp.de
Signed-off-by: Max Reitz <mreitz@redhat.com>
