ANDROID: net: ipv4: sysfs_net_ipv4: Add sysfs-based knobs for controlling TCP window size

Add a family of knobs to /sys/kernel/ipv4 for controlling the TCP window size:

	tcp_wmem_min
	tcp_wmem_def
	tcp_wmem_max
	tcp_rmem_min
	tcp_rmem_def
	tcp_rmem_max

This six values mirror the sysctl knobs in /proc/sys/net/ipv4/tcp_wmem and
/proc/sys/net/ipv4/tcp_rmem.

Sysfs, unlike sysctl, allows us to set and manage the files' permissions and
owners.

Signed-off-by: Robert Love <rlove@google.com>