sysfs: fix sysfs lockdep warning in mlx4 code

This fixes a sysfs lockdep warning in the mlx4 code.

Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Eric Biederman <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>