[nm_open] fix segfault when netmap_buf_size > 12928
[nm_open] fix segfault when netmap_buf_size > 12928

TODO: check if memsize (mmap len parameter) is too
	large (or there is overflow)

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