x86/mm/pti: Fix PTI comment in entry_SYSCALL_64()

The comment is confusing since the path is taken when
CONFIG_PAGE_TABLE_ISOLATION=y is disabled (while the comment says it is not
taken).

Signed-off-by: Nadav Amit <namit@vmware.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: nadav.amit@gmail.com
Link: http://lkml.kernel.org/r/20180209170638.15161-1-namit@vmware.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/entry_64.S

@@ -213,7 +213,7 @@ ENTRY(entry_SYSCALL_64)
 
 	swapgs
 	/*
-	 * This path is not taken when PAGE_TABLE_ISOLATION is disabled so it
+	 * This path is only taken when PAGE_TABLE_ISOLATION is disabled so it
 	 * is not required to switch CR3.
 	 */
 	movq	%rsp, PER_CPU_VAR(rsp_scratch)