powerpc: Fix bootmem reservation on uninitialized node

careful_allocation() was calling into the bootmem allocator for
nodes which had not been fully initialized and caused a previous
bug:  http://patchwork.ozlabs.org/patch/10528/

  So, I merged a
few broken out loops in do_init_bootmem() to fix it.  That changed
the code ordering.

I think this bug is triggered by having reserved areas for a node
which are spanned by another node's contents.  In the
mark_reserved_regions_for_nid() code, we attempt to reserve the
area for a node before we have allocated the NODE_DATA() for that
nid.  We do this since I reordered that loop.  I suck.

This is causing crashes at bootup on some systems, as reported
by Jon Tollefson.

This may only present on some systems that have 16GB pages
reserved.  But, it can probably happen on any system that is
trying to reserve large swaths of memory that happen to span other
nodes' contents.

This commit ensures that we do not touch bootmem for any node which
has not been initialized, and also removes a compile warning about
an unused variable.

Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>