lmb: Fix reserved region handling in lmb_enforce_memory_limit().

The idea of the implementation of this fix is from Michael Ellerman.

This function has two loops, but they each interpret the memory_limit
value differently.  The first loop interprets it as a "size limit"
whereas the second loop interprets it as an "address limit".

Before the second loop runs, reset memory_limit to lmb_end_of_DRAM()
so that it all works out.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Michael Ellerman <michael@ellerman.id.au>