mem: fix deadlock on secondary allocation

mem: fix deadlock on secondary allocation

Previous fix used `rte_malloc_heap_socket_is_external()` to check if the
heap was an external heap. However, that API is thread-safe, and when
we're inside the allocation process, we're already write-locked, so
calling `rte_malloc_heap_socket_is_external()` will result in a
deadlock followed by a timeout.

Fix it by replacing the API call with a check against maximum number of
NUMA nodes, because external heaps always have higher socket ID's.

Fixes: 7ac31e82bc8f ("mem: improve parameter checking on memory hotplug")

Reported-by: Jim Harris <james.r.harris@intel.com>
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>