Allocate the device buffer only if the protocol really need to send data.
not all protocols require the driver to prepare additional iob before
sending, especially UDP, each iob reserves l2/l3 header in advance
after prepare write buffer, net device could reuse this entry to send directly
Signed-off-by: chao an <anchao@xiaomi.com>
tcp_timer: eliminated false decrements of conn->timer in case of multiple network adapters.
The false timer decrements sometimes provoked TCP spurious retransmissions due to premature timeouts.
