btrfs: rename delete_unused_bgs_mutex to reclaim_bgs_lock

As a preparation for extending the block group deletion use case, rename
the unused_bgs_mutex to reclaim_bgs_lock.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>