Enhance the driver to handle whatever maximum segment number the host
tells us to handle.  Do to this, we need to allocate the scatterlist
dynamically.

We set max_phys_segments and max_hw_segments to the same value (1 if
the host doesn't tell us, since that's safest and all known hosts do
tell us).

Note that kmalloc'ing the structure for large sg_elems might be
problematic: the fix for this is sg_table, but that requires more
work.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>