btrfs: avoid overflow when sector_t is 32 bit

Jean-Denis Girard noticed commit c821e7f3 "pass bytes to
btrfs_bio_alloc" (https://patchwork.kernel.org/patch/9763081/)
introduces a regression on 32 bit machines.
When CONFIG_LBDAF is _not_ defined (CONFIG_LBDAF == Support for large
(2TB+) block devices and files) sector_t is 32 bit on 32bit machines.

In the function submit_extent_page, 'sector' (which is sector_t type) is
multiplied by 512 to convert it from sectors to bytes, leading to an
overflow when the disk is bigger than 4GB (!).

I added a cast to u64 to avoid overflow.

Fixes: c821e7f3

 ("btrfs: pass bytes to btrfs_bio_alloc")
CC: stable@vger.kernel.org # 4.13+
Signed-off-by: Goffredo Baroncelli <kreijack@inwind.it>
Tested-by: Jean-Denis Girard <jd.girard@sysnux.pf>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>