[LMB]: Fix lmb_add_region if region should be added at the head

We introduced a bug in fixing lmb_add_region to handle an initial
region being non-zero.  Before that fix it was impossible to insert a
region at the head of the list since the first region always started
at zero.

Now that its possible for the first region to be non-zero we need to
check to see if the new region should be added at the head and if so
actually add it.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>