x86-64: Clear a 64-bit FS/GS base on fork if selector is nonzero

When we do a thread switch, we clear the outgoing FS/GS base if the
corresponding selector is nonzero.  This is taken by __switch_to() as
an entry invariant; it does not verify that it is true on entry.
However, copy_thread() doesn't enforce this constraint, which can
result in inconsistent results after fork().

Make copy_thread() match the behavior of __switch_to().

Reported-and-tested-by: Samuel Thibault <samuel.thibault@inria.fr>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
LKML-Reference: <4BD1E061.8030605@zytor.com>
Cc: <stable@kernel.org>