[PATCH] i386: Fix pack_descriptor()

Fix pack_descriptor:
 1. flags are bits 20-23 in the high word
 2. limit's 4 msb are bits 16-19 in the high word

These haven't mattered so far, because all users have had small limits
and a flags setting of 0.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Andi Kleen <ak@suse.de>
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