x86, vsyscall: fix the oops crash with __pa_vsymbol()

Appended patch fixes an oops while changing the vsyscall sysctl.
I am sure no one tested this code before integrating into mainline :(

BTW, using ioremap() in vsyscall_sysctl_change() to get the virtual
address of a kernel symbol sounds like an over kill.. I wonder if we
can define a simple __va_vsymbol() which will return directly the
kernel direct mapping. comments in the code which says gcc has trouble
with __va(__pa()) sounds bogus to me. __pa() on a vsyscall address will
not work anyhow :(

And also, the whole nop out syscall in vsyscall page infrastructure
(vsyscall_sysctl_change()) is added to make some attacks difficult,
and yet I don't see this nop out being done by default. This area
requires more cleanups?

Fix an oops with __pa_vsymbol(). VSYSCALL_FIRST_PAGE is a fixmap index.
We want the starting virtual address of the vsyscall page and not the index.

[ mingo: arch/x86 adaptation ]

Reported-by: Yanmin Zhang <yanmin.zhang@intel.com>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/vsyscall_64.c

@@ -48,7 +48,7 @@
 	({unsigned long v;  		\
 	extern char __vsyscall_0; 	\
 	  asm("" : "=r" (v) : "0" (x)); \
-	  ((v - VSYSCALL_FIRST_PAGE) + __pa_symbol(&__vsyscall_0)); })
+	  ((v - VSYSCALL_START) + __pa_symbol(&__vsyscall_0)); })
 
 /*
  * vsyscall_gtod_data contains data that is :