MFC r271794: When updating device media size use cached cdevsw pointer.

Using pointer from the cdev directly is dangerous since we have no
reference on it, and it may change any time.  That caused panic if
device has gone.

While there, report capacity change only if it really changed.

Approved by:	re (dephij)
MFC r271797:  Make kernel to update LUN size from the backing storage
on configuration reload also if that size was not specified in the new
configuration.

Previously it happened only if size was explicitly changed in config.

Approved by:	re (delphij)