Btrfs: find_next_devid: root -> fs_info

find_next_devid() knows which root to search, so it should take an
fs_info instead of an arbitrary root.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>