driver core: make sysfs_dev_block_kobj static

Nothing outside of drivers/base/core.c uses sysfs_dev_block_kobj, so
make it static and document what it is used for so we remember it the
next time we touch it 15 years from now.

Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230331093318.82288-6-gregkh@linuxfoundation.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>