quota: Don't store flags for v2 quota format

Currently, v2 quota format blindly stored flags from in-memory dqinfo on
disk, although there are no flags supported. Since it is stupid to store
flags which have no effect, just store 0 unconditionally and don't
bother loading it from disk.

Note that userspace could have stored some flags there via Q_SETINFO
quotactl and then later read them (although flags have no effect) but
I'm pretty sure noone does that (most definitely quota-tools don't and
quota interface doesn't have too much other users).

Signed-off-by: Jan Kara <jack@suse.cz>