Fix bug in MTT allocation in mem-free mode.

I misunderstood the MTT size value returned by the firmware -- it is really
the size of a single MTT entry, since mem-free mode does not segment the MTT
as the original firmware did.  This meant that our MTT addresses ended up
being off by a factor of 8.  This meant that our MTT allocations might
overlap, and so we could overwrite and corrupt earlier memory regions when
writing new MTT entries.

We fix this by always using our 64-byte MTT segment size.  This allows some
simplification of the code as well, since there's no reason to put the MTT
segment size in a variable -- we can always use our enum value directly.
Signed-off-by: Roland Dreier <roland@topspin.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>