block: move disk invalidation from del_gendisk() into a helper

Move the disk / partition invalidation into a helper. This will make
reading del_gendisk easier to read, in preparation for adding support
to add error handling later on register_disk() and to later share more
code with del_gendisk.

This change has no functional changes.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>