x86: Default to vsyscall=native for now

This UML breakage:

  linux-2.6.30.1[3800] vsyscall fault (exploit attempt?) ip:ffffffffff600000 cs:33 sp:7fbfb9c498 ax:ffffffffff600000 si:0 di:606790
  linux-2.6.30.1[3856] vsyscall fault (exploit attempt?) ip:ffffffffff600000 cs:33 sp:7fbfb13168 ax:ffffffffff600000 si:0 di:606790

Is caused by commit 3ae36655 ("x86-64: Rework vsyscall emulation and add
vsyscall= parameter") - the vsyscall emulation code is not fully cooked
yet as UML relies on some rather fragile SIGSEGV semantics.

Linus suggested in https://lkml.org/lkml/2011/8/9/376 to default
to vsyscall=native for now, this patch implements that.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Acked-by: Andrew Lutomirski <luto@mit.edu>
Cc: H. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/r/20111005214047.GE14406@localhost.pp.htv.fi
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Documentation/kernel-parameters.txt

@@ -2706,10 +2706,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			functions are at fixed addresses, they make nice
 			targets for exploits that can control RIP.
 
-			emulate     [default] Vsyscalls turn into traps and are
-			            emulated reasonably safely.
+			emulate     Vsyscalls turn into traps and are emulated
+			            reasonably safely.
 
-			native      Vsyscalls are native syscall instructions.
+			native      [default] Vsyscalls are native syscall
+			            instructions.
 			            This is a little bit faster than trapping
 			            and makes a few dynamic recompilers work
 			            better than they would in emulation mode.