quota: add missing lock into __dquot_transfer()

Lock dq_dqb_lock around dquot_decr_inodes()

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Fixes: 7b9ca4c6 ("quota: Reduce contention on dq_data_lock")
Signed-off-by: Jan Kara <jack@suse.cz>