Partially saved extended state must be handled always, i.e. for both
fpu-owned context, and for pcb-saved one. More, the XSAVE could do
partial save, same as XSAVEOPT, so qualifier for the handler should be
use_xsave and not use_xsaveopt.

Since xsave_area_desc is now needed regardless of the XSAVEOPT use,
remove the write-only use_xsaveopt variable.

In collaboration with: jhb
MFC after: 1 week