x86-64: Improve vsyscall emulation CS and RIP handling

Three fixes here:
 - Send SIGSEGV if called from compat code or with a funny CS.
 - Don't BUG on impossible addresses.
 - Add a missing local_irq_disable.

This patch also removes an unused variable.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/6fb2b13ab39b743d1e4f466eef13425854912f7f.1310563276.git.luto@mit.edu
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>