reiserfs: Fix resuming of quotas on remount read-write

When quota was suspended on remount-ro, finish_unfinished() will try to turn
it on again (which fails) and also turns the quotas off on exit. Fix the
function to check whether quotas are already on at function entry and do
not turn them off in that case.

CC: reiserfs-devel@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>