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
45bc92633d