ipv4/tcp: Use local variable for tcp_md5_addr

Extract the typecast to (union tcp_md5_addr *) to a local variable
rather than the current long, inline declaration with function calls.

No functional change intended.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>