btrfs-progs: subvol: let sync check only current deletions

btrfs-progs: subvol: let sync check only current deletions

So far the subvol sync command takes a shortcut and looks if there are
any deleted subvols at all. It does not print the deleted subvolumes as
they get cleaned. Arguably this is what the user would like to see and
has to do

 $ btrfs subvol sync /path $(btrfs subvol list -d /path | "extract the ids")

to see the progress.

Make it look for all currently deleted subvolumes automatically and
print the progress as if the ids were listed manually.

This is a slight change in the semantics of the command. Previously, any
new subvol deletion would prevent subvol sync to return. To simulate the
old behaviour, run 'subvol sync' in a loop until it returns 0.

Signed-off-by: David Sterba <dsterba@suse.cz>