lib/string_helpers: string_get_size() now returns characters wrote

lib/string_helpers: string_get_size() now returns characters wrote

printbuf now needs to know the number of characters that would have been
written if the buffer was too small, like snprintf(); this changes
string_get_size() to return the the return value of snprintf().

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>