extmem: use a bitmap to record skipped objects
extmem: use a bitmap to record skipped objects

Pre-allocated objects that cross non-contiguous pages cannot be used.
The previous code skipped these objects may setting their vaddr to NULL
in the lut, but this created bugs in the mmap code.

luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap