flowcache: make flow_key_size() return "unsigned int"

Flow keys aren't 4GB+ numbers so 64-bit arithmetic is excessive.

Space savings (I'm not sure what CSWTCH is):

	add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-48 (-48)
	function                                     old     new   delta
	flow_cache_lookup                           1163    1159      -4
	CSWTCH                                     75997   75953     -44

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>