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.
Approved by: re (gjb)
Reported by: markj
Reviewed by: markj
MFC after: 1 week
(cherry picked from commit ee7ffaa2e6e08b63efb4673610875d40964d5058)
(cherry picked from commit 3e4127f8f2933029034ac618a0013f434cb4a420)