Some kernel modules call device_del() from their module exit code and
schedule asynchronous work from inside the .release callback without waiting
until that callback has finished. As an example, many SCSI LLD drivers call
scsi_remove_host() from their module exit code. scsi_remove_host() may
invoke scsi_device_dev_release_usercontext() asynchronously.
scsi_device_dev_release_usercontext() uses the host template pointer and
that pointer usually exists in static storage in the SCSI LLD. Support
using the module reference count to keep the module around until
asynchronous module exiting has completed by waiting in the delete_module()
system call until the module reference count drops to zero.