btrfs: constify ulist parameter of ulist_next()

The ulist_next() iterator function does not need to change the given ulist
so make it const. This will allow the next patch in the series to pass a
ulist to a function that does not need, and should not, modify the ulist.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>