ip: Fix size_columns() for very large values

ip: Fix size_columns() for very large values

For values near the 64-bit boundary, the iterative application of
powi *= 10 causes powi to overflow without the termination condition of
powi >= val having ever been satisfied. Instead, when determining the
length of the number, iterate val /= 10 and terminate when it's a single
digit.

Fixes: 49437375b6c1 ("ip: dynamically size columns when printing stats")
CC: Tariq Toukan <tariqt@nvidia.com>
CC: Itay Aveksis <itayav@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>