netmap: fix issues in nm_os_extmem_create()

• Call vm_object_reference() before vm_map_lookup_done().
• Use vm_mmap_to_errno() to convert vm_map_* return values to errno.
• Fix memory leak of e->obj.

Reported by: markj
Reviewed by: markj
MFC after: 1 week

(cherry picked from commit ee7ffaa2e6e08b63efb4673610875d40964d5058)