quota: Properly return errors from dquot_writeback_dquots()

Due to a switched left and right side of an assignment,
dquot_writeback_dquots() never returned error. This could result in
errors during quota writeback to not be reported to userspace properly.
Fix it.

CC: stable@vger.kernel.org
Coverity-id: 1226884
Signed-off-by: Jan Kara <jack@suse.cz>