quota: special case Q_SYNC without device name

The Q_SYNC command can be called without the path to a device, in which case
it iterates over all superblocks.  Special case this variant directly in
sys_quotactl so that the other code always gets a superblock and doesn't
need to deal with this case.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>