kerndat: tcp_max_rshare -- Shrink default size

In commit 664659a0 we defined max possible
values for TCP queues but on low memory
machines (or on machinese whith custom limits
set via sysctl) this trick doesn't work well
all the time. We have to fetch the allowed values
from /proc/sys/net/ipv4/tcp_[wmem|rmem] which
might be simply missing on the environment
we run.

So lets stick with really small values here
got from kernel source code so instead of failing
we would be able to restore in a bit "slower"
than usual.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>