inet: tcp -- Find size of max memory allowed to restore TCP data

The maximal size which may be used in the kernel for sending TCP data
on restore is varies depending on how many memory installed on the
system, moreover the memory allocated for "read queue" is bigger than
used for "write queue". Thus when we checkpointed a big slab of data
we need to figure out which size is allowed for sending data on restore.

For this we read /proc/sys/net/ipv4/tcp_[wmem|rmem] on restore and calculate
the size needed, then we simply chop data to segements and send it
in a loop.

Typical output on restore is something like

 | (00.013001)  30110: TCP queue memory limits are 2097152:3145728

https://bugzilla.openvz.org/show_bug.cgi?id=2751



[xemul: moved stuff to kerndat.c]

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