Strfree() should call kfree() not kmem_free()

Strfree() should call kfree() not kmem_free()

Using kmem_free() results in deducting X bytes from the memory
accounting when --enable-debug is set.  Unfortunately, currently
the counterpart kmem_asprintf() and friends do not properly
account for memory allocated, so we must do the same on free.
If we don't then we end up with a negative number of lost bytes
reported when the module is unloaded.

A better long term fix would be to add the accounting in to the
allocation side but that's a project for another day.