KVM: vmx: compute need to reload FS/GS/LDT on demand

Remove fs_reload_needed and gs_ldt_reload_needed from host_state
and instead compute whether we need to reload various state at
the time we actually do the reload.  The state that is tracked
by the *_reload_needed variables is not any more volatile than
the trackers themselves.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>