[PATCH] UML: fix wall_to_monotonic initialization

Change a variable from unsigned to signed in order to get sign-extension
when the thing is negated.  Without this, uptime is horribly confused.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>