btrfs-progs: subvol: fix subvol del --commit-after

btrfs-progs: subvol: fix subvol del --commit-after

Fix 'subvolume delete --commit-after' to work properly:
- SYNC ioctl will be issued even when last delete fails
- SYNC ioctl will be issued on each file system only once in the end

To achieve this, get_fsid() and add_seen_fsid() are called after each
delete to keep only one fd for each fs.

In the end, seen_fsid_hash will be traversed and SYNC is issued on each
fs.

Signed-off-by: Tomohiro Misono <misono.tomohiro@jp.fujitsu.com>
Reviewed-by: Qu Wenruo <quwenruo.btrfs@gmx.com>
Signed-off-by: David Sterba <dsterba@suse.com>