btrfs: consolidate btrfs_previous_item ret val handling in btrfs_shrink_device

Instead of having three 'if' to handle non-NULL return value consolidate
this in one 'if (ret)'. That way the code is more obvious:

 - Always drop delete_unused_bgs_mutex if ret is not NULL
 - If ret is negative -> goto done
 - If it's 1 -> reset ret to 0, release the path and finish the loop.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>