x86: Make sure get_user_desc() doesn't sign extend.

The current implementation of get_user_desc() sign extends the return
value because of integer promotion rules.  For the most part, this
doesn't matter, because the top bit of base2 is usually 0.  If, however,
that bit is 1, then the entire value will be 0xffff...  which is
probably not what the caller intended.

This patch casts the entire thing to unsigned before returning, which
generates almost the same assembly as the current code but replaces the
final "cltq" (sign extend) with a "mov %eax %eax" (zero-extend).  This
fixes booting certain guests under KVM.

Signed-off-by: Chris Lalancette <clalance@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>