Allow mlx4 devices to switch from Ethernet to Infiniband (and vice versa):
- Fix sysctl wrapper for sysfs attributes to properly handle new string
  values similar to sysctl_handle_string() (only copyin the user's
  supplied length and nul-terminate the string).
- Don't check for a trailing newline when evaluating the desired operating
  mode of a mlx4 device.

PR:		kern/179999
Submitted by:	Shahar Klein <shahark@mellanox.com>
MFC after:	1 week