BUG/MEDIUM: counters: ensure that src_{inc,clr}_gpc0 creates a missing entry

BUG/MEDIUM: counters: ensure that src_{inc,clr}_gpc0 creates a missing entry

During 1.5-dev20 there was some code refactoring to make the src_* fetch
function use the same code as sc_*. Unfortunately this introduced a
regression where src_* doesn't create an entry anymore if it does not
exist in the table. The reason is that smp_fetch_sc_stkctr() only calls
stktable_lookup_key() while src_inc_*/src_clr_* used to make use of
stktable_update_key() which additionally create the entry if it does
not exist.

There's no point modifying the common function for these two exceptions,
so instead we now have a function dedicated to the creation of this entry
for src_* only. It is called when the entry didn't exist, so that requires
minimal modifications to existing code.

Thanks to Thierry Fournier for helping diagnose the issue.

This fix must be backported to 1.5.