mmap-alloc: tweak a comment on ppc64

The comment I put in mmap-alloc to document the ppc64 rules
refers to the previous revision of the patch:
we don't look at memory alignment anymore, we check
the fs from which the fd is mapped, instead.

It's also not clear what does "in this case" refer
to, rearrange text to make it clearer.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>




