[PATCH] quota: fix possible oops on quotaoff

Remove dquot structures from quota file on quotaon - quota code does not
expect them to be there.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>