ext3: quota_write cross block boundary behaviour

We always assume what dquot update result in changes in one data block
But ext3_quota_write() function may handle cross block boundary writes
In fact if this ever happen it will result in incorrect journal credits
reservation. And later bug_on triggering. As soon this never happen the
boundary cross loop is NOOP. In order to make things straight
let's remove this loop and assert cross boundary condition.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Jan Kara <jack@suse.cz>