x86/extable: Add a comment about early exception handlers

Borislav asked for a comment explaining why all exception handlers are
allowed early.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: KVM list <kvm@vger.kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: xen-devel <Xen-devel@lists.xen.org>
Link: http://lkml.kernel.org/r/5f1dcd6919f4a5923959a8065cb2c04d9dac1412.1459784772.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>