encoding: make .size() to __u32 cast explicit

encoding: make .size() to __u32 cast explicit

Caught by clang warning that this is a conversion
from "unsigned long" to "unsigned int" which can
lose precision.  However, this is the conversion
we want because our serialization format defines
sizes as 4 bytes.

Signed-off-by: John Spray <john.spray@inktank.com>