vppinfra: preserve total_cpu_time across clock freq adjustment

vppinfra: preserve total_cpu_time across clock freq adjustment

If clib_time_verify_frequency() adjusts the clock frequency, transform
total_cpu_time to the new time coordinate space. Otherwise, we break
comparisons with previous clib_time_now() value.

Without this correction, time jumps in one direction or the other
depending on the sign of the frequency change. Reasonably harmless in
most cases, but under perfect storm conditions the wheels fall off.

Type: fix

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I21802c2630e2c87ff817cd732b7d78bc022cd2d7