fdisk: use a single variable for the current disklabel

fdisk: use a single variable for the current disklabel

Using a variable for each supported partition table type doesn't seem necessary.

This fixes also a minor bug in switching from SGI label to SUN label: the expert
menu isn't available as sgi_label variable remains set to true.

Code a bit more clear as *_label names have similarity with names such as
"sgilabel", "struct sun_label".

Signed-off-by: Francesco Cosoleto <cosoleto@gmail.com>