quota: Remove BUG_ON in dquot_load_quota_sb()

The case when someone passes DQUOT_SUSPENDED flag to
dquot_load_quota_sb() is easy to handle. So just WARN_ON_ONCE and bail
with error if that happens instead of calling BUG_ON which is likely to
lockup the machine.

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