struct flow_entry does not need field in_use

[AltraMayor]

The field in_use of struct flow_entry can be replaced with the following inline function:

static inline bool
flow_entry_in_use(const struct flow_entry *fe)
{
	return fe->grantor_fib != NULL;
}

The field grantor_fib must be set for GK blocks to know how to forward packets to Grantor servers or the actual destination, so it is always set for entries in use.

Dropping field in_use is just one more step to streamline struct flow_entry.