alpha: fix lazy-FPU mis(merged/applied/whatnot)

Looks like a braino that used to be fixed in e.g. #next.alpha
had gotten into alpha.git cherry-picked version of that patch.

Sure, alpha has no preempt, but preempt_enable() in place of
preempt_disable() is actively confusing the readers...

Other than that, the cherry-picked variant matches what I have.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>