x86/mm/64: Remove the last VM_BUG_ON() from the TLB code

Let's avoid hard-to-diagnose crashes in the future.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/f423bbc97864089fbdeb813f1ea126c6eaed844a.1508000261.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>