flowcache: make flow_cache_hash_size() return "unsigned int"

Hash size can't negative so "unsigned int" is logically correct.

Propagate "unsigned int" to loop counters.

Space savings:

	add/remove: 0/0 grow/shrink: 2/2 up/down: 6/-18 (-12)
	function                                     old     new   delta
	flow_cache_flush_tasklet                     362     365      +3
	__flow_cache_shrink                          333     336      +3
	flow_cache_cpu_up_prep                       178     171      -7
	flow_cache_lookup                           1159    1148     -11

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