charset: Add support for calculating bytes occupied by a u16 string

The current code uses 'u16_strlen(x) + 1) * sizeof(u16)' in various
places to calculate the number of bytes occupied by a u16 string.
Let's introduce a wrapper around this. This wrapper is used on following
patches

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>