[kernel][iob] Mapping requires ZX_RIGHT_MAP

Require that to create mappings from an iob, using zx_vmar_map_iob, the
handle to the iob itself must have the ZX_RIGHT_MAP right, not just the
underlying region itself.

Fixed: 517306781
Change-Id: I99c1d08867c97930a5bfb00ed64cda9c5270bb2c
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1633794
Reviewed-by: James Robinson <jamesr@google.com>
Fuchsia-Auto-Submit: Adrian Danis <adanis@google.com>
Commit-Queue: Adrian Danis <adanis@google.com>