sendfile uses sendpage() and all data are splitted on pages and
a new skb is allocated for each page. It creates a big overhead on SNDBUF.

sendfile() isn't suatable for DGRAM sockets, because message boundaries
should be saved.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>