[5.2 REGRESSION] Generic vDSO breaks seccomp-enabled userspace on i386

[Sean Christopherson]

The generic vDSO implementation, starting with commit

   7ac870747988 ("x86/vdso: Switch to generic vDSO implementation")

breaks seccomp-enabled userspace on 32-bit x86 (i386) kernels.  Prior to
the generic implementation, the x86 vDSO used identical code for both
x86_64 and i386 kernels, which worked because it did all calcuations using
structs with naturally sized variables, i.e. didn't use __kernel_timespec.

The generic vDSO does its internal calculations using __kernel_timespec,
which in turn requires the i386 fallback syscall to use the 64-bit
variation, __NR_clock_gettime64.

Using __NR_clock_gettime64 instead of __NR_clock_gettime breaks userspace
applications that use seccomp filtering to block syscalls, as applications
are completely unaware of the newly added of __NR_clock_gettime64, e.g.
sshd gets zapped on syscall(403) when attempting to ssh into the system.

I can fudge around the issue easily enough, but I have no idea how to fix
this properly without duplicating __cvdso_clock_gettime, do_hres, etc...,
especially now that the i386 vDSO exposes __vdso_clock_gettime64().