soreuseport: use "unsigned int" in __reuseport_alloc()

Number of sockets is limited by 16-bit, so 64-bit allocation will never
happen.

16-bit ops are the worst code density-wise on x86_64 because of
additional prefix (66).

Space savings:

	add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-3 (-3)
	function                                     old     new   delta
	reuseport_add_sock                           539     536      -3

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