Can be invoked from non-BH context.

Based upon a patch by Eric Dumazet.

Fixes: f19c29e3 ("tcp: snmp stats for Fast Open, SYN rtx, and data pkts")
Reported-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>