BUG/MEDIUM: spoe: arg len encoded in previous frag frame but len changed

BUG/MEDIUM: spoe: arg len encoded in previous frag frame but len changed

Fragmented arg will do fetch at every encode time, each fetch may get
different result if SMP_F_MAY_CHANGE, for example res.payload, but
the length already encoded in first fragment of the frame, that will
cause SPOA decode failed and waste resources.

This patch must be backported to 1.9 and 1.8.