Fix lockdep warning on insmod

sysfs_attr_init() is required to make lockdep happy for dynamically
allocated sysfs attributes. This fixed #8868 on Fedora 29 running
kernel-debug.

This requirement was introduced in 2.6.34.
See include/linux/sysfs.h for what it actually does.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes #8868
Closes #8884