MFC 253048,253423,253449,253653,253774,253785:
- Allow mlx4 devices to switch between Ethernet and Infiniband:
  - Fix sysfs attribute handling by using sysctl_handle_string() and
    properly handling trailing newlines in attribute values.
  - Remove check forbidding requests that would result in one port being
    set to Ethernet and the subsequent port being set to IB.
- Avoid trashing IP fragments by correctly managing hardware checksumming.
- Fix panics when downing or unloading the mlx4 driver.

PR:		kern/179999, kern/174213, kern/180430, kern/180791