x86_64/vsyscall: Restore orig_ax after vsyscall seccomp

The vsyscall emulation code sets orig_ax for seccomp's benefit,
but it forgot to set it back.

I'm not sure that this is observable at all, but it could cause
confusion to various /proc or ptrace users, and it's possible
that it could cause minor artifacts if a signal were to be
delivered on return from vsyscall emulation.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/cdc6a564517a4df09235572ee5f530ccdcf933f7.1415144089.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>