btrfs: use single return variable in btrfs_find_orphan_roots()

btrfs: use single return variable in btrfs_find_orphan_roots()

We use both 'ret' and 'err' which is a pattern that generates confusion
and resulted in subtle bugs in the past. Remove 'err' and use only 'ret'.
Also move simplify the error flow by directly returning from the function
instead of breaking of the loop, since there are no resources to cleanup
after the loop.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>