Rework the previous fix for the IB vs Ethernet sysctl handler to be more

Rework the previous fix for the IB vs Ethernet sysctl handler to be more
generic and apply to all sysfs attributes:
- Use sysctl_handle_string() instead of reimplementing it.
- Remove trailing newline from the current value before passing it to
  userland and append a newline to the new string value before passing it
  to the attribute's store function.
- Don't leak the temporary buffer if the first error check triggers.
- Revert earlier change to mlx4 port mode handler.

PR:		kern/174213
Submitted by:	Garrett Cooper
Reviewed by:	Shakar Klein @ Mellanox
MFC after:	1 week