socktop: Make sure *_str are always known as strings

socktop: Make sure *_str are always known as strings

The new removal of unreachable code meant socktop might remove its whole
filter setup, and all the related variables are left with unknown type.
They would typically be pruned away just fine, but socktop also has a
probe never with print statements to avoid autoprint.  Changing those to
log() lets stap always know they are strings.