ext4: allocate stripe-multiple IOs on stripe boundaries

For some reason, today mballoc only allocates IOs which are exactly
stripe-sized on a stripe boundary.  If you have a multiple (say, a
128k IO on a 64k stripe) you may end up unaligned.

It seems to me that a simple change to align stripe-multiple IOs
on stripe boundaries would be a very good idea, unless this breaks
some other mballoc heuristic for some reason...

Reported-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>