looking at ext3_xattr_block_set() [fs/ext3/xattr.c] ...
I see that

                                error = -EDQUOT;
                                if (DQUOT_ALLOC_BLOCK(inode, 1))
                                        goto cleanup;

allocates a quota block, but right after that several
error echecks happen ...

                                if (error)
                                        goto cleanup;

and I don't see any DQUOT_FREE_BLOCK() in the errorpath

cleanup:
        if (ce)
                mb_cache_entry_release(ce);
        brelse(new_bh); 
        if (!(bs->bh && s->base == bs->bh->b_data))
                kfree(s->base);

        return error;

I'd suggest the attached fix.
Acked-by: Jan Kara <jack@suse.cz>
Acked-by: Andreas Gruenbacher <agruen@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>