x86/vdso: Simplify the invalid vclock case

x86/vdso: Simplify the invalid vclock case

The code flow for the vclocks is convoluted as it requires the vclocks
which can be invalidated separately from the vsyscall_gtod_data sequence to
store the fact in a separate variable. That's inefficient.

Restructure the code so the vclock readout returns cycles and the
conversion to nanoseconds is handled at the call site.

If the clock gets invalidated or vclock is already VCLOCK_NONE, return
U64_MAX as the cycle value, which is invalid for all clocks and leave the
sequence loop immediately in that case by calling the fallback function
directly.

This allows to remove the gettimeofday fallback as it now uses the
clock_gettime() fallback and does the nanoseconds to microseconds
conversion in the same way as it does when the vclock is functional. It
does not make a difference whether the division by 1000 happens in the
kernel fallback or in userspace.

Generates way better code and gains a few cycles back.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Matt Rickard <matt@softrans.com.au>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: devel@linuxdriverproject.org
Cc: virtualization@lists.linux-foundation.org
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Juergen Gross <jgross@suse.com>
Link: https://lkml.kernel.org/r/20180917130707.657928937@linutronix.de