flowcache: more "unsigned int"

Make ->hash_count, ->low_watermark and ->high_watermark unsigned int
and propagate unsignedness to other variables.

This change doesn't change code generation because these fields aren't
used in 64-bit contexts but make it anyway: these fields can't be
negative numbers.

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