x86/mm: Convert unreachable() to BUG()

Commit 2190966f ("x86: Convert unreachable() to BUG()") missed
one.

And after commit 06e24745 ("objtool: Remove
annotate_{,un}reachable()") the invalid use of unreachable()
(rightfully) triggers warnings:

  vmlinux.o: warning: objtool: page_fault_oops() falls through to next function is_prefetch()

Fixes: 2190966f ("x86: Convert unreachable() to BUG()")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20241216093215.GD12338@noisy.programming.kicks-ass.net