Btrfs: make sure we update latest_bdev

When we are setting up the mount, we close all the
devices that were not actually part of the metadata we found.

But, we don't make sure that one of those devices wasn't
fs_devices->latest_bdev, which means we can do a use after free
on the one we closed.

This updates latest_bdev as it goes.

Signed-off-by: Chris Mason <chris.mason@oracle.com>