allocman: fix use of ALLOCMAN_NO_PADDR

Throughout allocman, various parts of the code for allocating a
specific paddr would check that the paddr fell within the bounds of a
given untyped node. Unfortunately, in some cases ALLOCMAN_NO_PADDR +
node->size would overlap with the paddr and cause a false positive -
allocman would *think* that a given node contains the request paddr,
when in fact it didn't.

Eventually, everything would fall apart: ALLOCMAN_NO_PADDR+node->size
would not be large enough to reach the paddr, and allocman would end
up with a NULL node where it wasn't expecting one.

Fix this issue by avoiding nodes with paddr == ALLOCMAN_NO_PADDR
whenever we're searching through node lists, thus preventing
such 'false positive' situations from occurring.