x86: modify write_ldt function

This patch modifies the write_ldt() function to make use
of the new struct desc_struct instead of entry_1 and entry_2
entries

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>