x86: remove extra barriers from load_gs_base()

Impact: optimization

mb() generates an mfence instruction, which is not needed here.  Only
a compiler barrier is needed, and that is handled by the memory clobber
in the wrmsrl function.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>