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