Use g_resize_provider() to change the size of GEOM_DISK provider,
when it is being opened. This should fix the possible loss of a resize
event when disk capacity changed.

Do not invoke resize method if geom is being withered.

Do not invoke resize event if initial disk size is zero. Some disks
report the size only after first opening.  And due to the events are
asynchronous, some consumers can receive this event too late and
this confuses them. This partially restores previous behaviour, and
at the same time this should fix the problem, when already opened
provider loses resize event.