lib/vdso: Force inlining of __cvdso_clock_gettime_common()

When adding gettime64() to a 32 bit architecture (namely powerpc/32)
it has been noticed that GCC doesn't inline anymore
__cvdso_clock_gettime_common() because it is called twice
(Once by __cvdso_clock_gettime() and once by
__cvdso_clock_gettime32).

This has the effect of seriously degrading the performance:

Before the implementation of gettime64(), gettime() runs in:

  clock-gettime-monotonic-raw:	    vdso: 1003 nsec/call
  clock-gettime-monotonic-coarse:   vdso:  592 nsec/call
  clock-gettime-monotonic:          vdso:  942 nsec/call

When adding a gettime64() entry point, the standard gettime()
performance is degraded by 30% to 50%:

  clock-gettime-monotonic-raw:      vdso: 1300 nsec/call
  clock-gettime-monotonic-coarse:   vdso:  900 nsec/call
  clock-gettime-monotonic:          vdso: 1232 nsec/call

Adding __always_inline() to __cvdso_clock_gettime_common() regains the
original performance.

In terms of code size, the inlining increases the code size by only 176
bytes. This is in the noise for a kernel image.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/1ab6a62c356c3bec35d1623563ef9c636205bcda.1588079622.git.christophe.leroy@c-s.fr