x86: use symbolic name for VM86_SIGNAL when used as vm86 default return

This code has apparently used "0" and not VM86_SIGNAL since Linux
1.1.9, when Linus added VM86_SIGNAL to vm86.h. This patch changes the
code to use the symbolic name.

The magic 0 tripped me up in trying to extend the vm86(2) manpage to
actually explain vm86()'s interface -- my greps for VM86_SIGNAL came up
fruitless.

[ Impact: cleanup; no object code change ]

Signed-off-by: Samuel Bronson <naesten@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>